THE BIOLOGY OF THE LEAF

IN nursing homes and hospitals all plants and cut flowers are removed from the wards before nightfall, because of the interchange of gases that takes place between green plants and the atmosphere.

Always, every moment of the day and night, a plant is using up atmospheric oxygen and giving out carbon dioxide, for respiration never ceases until life itself comes to an end.

In daylight, in the case of green plants, another gaseous interchange is going on side by side with the respiratory process. This is the taking in of carbon dioxide and the subsequent return of oxygen, as a result of carbon assimilation.

In both processes gases pass in and out of the leaf by way of small apertures or mouths, called stomata. Actually to see these it is necessary to shave off the epidermis of a leaf, mount the thin layer on a slide, and examine it under the microscope. The mouths are then seen as gaps, each bounded by two sausage-shaped cells .

To prove, without seeing them, that apertures are present in the epidermis is quite a simple matter.

It is only necessary to boil some water in a beaker until all air-bubbles have escaped, then drop a leaf into the water.

The air inside the leaf becomes hot and expands so much that it escapes in tiny bubbles, more particularly from the morphologically lower surface of the leaf, where the veins are most prominent. Not only, then, must gaps be present in the epidermis to make possible this escape, but they are evidently much more numerous on the under than on the upper surface.

To prove that air surrounding a leaf can pass into and through it, needs a little patience.

A large firm leaf, like that of the Laurel, should be fixed into a glass tube of relatively small bore, the connection between leaf stalk and tube being made air-tight with paraffin wax. The tube, which is drawn out to an open point at the other end, passes through one hole of a rubber stopper into a flask which contains a little water into which the point of the tube must dip. Passing through a second hole of the stopper is a short right-angled glass tube with a piece of rubber tubing fixed over its free end, so that it can be closed by a clip at will.

Some of the air that is in the flask above the water can be drawn out by suction through the right-angled tube. As soon as the lips are removed the rubber tubing must be fastened by the clip.

If the experiment is successful, that is, if all connections are air-tight, bubbles escape through the water from the pointed end of the tube that holds the leaf and continue to pass for some time .

By suction air has been removed from the flask. Air from the leaf has taken its place. To make good the loss inside the leaf some of the air of the room has passed into it through the stomata of the epidermis.

Such, then, is the course of gases that enter the leaf, both in respiration and in carbon assimilation.

The leaf area of a plant is so great in comparison with that of other members that it is natural to look upon leaves as being the chief organs concerned in the work of respiration. They certainly do respire freely, but so do all other parts of a plant – those that are not green as well as those that contain chlorophyll, those that are below the ground as well as those that project above it.

In spite of all this co-operative breathing, and in spite of the fact that, speaking generally, leaves only are concerned in carbon assimilation, the volume of oxygen given out in- the daytime, as a result of carbon assimilation, is greater than the volume of carbon dioxide given out, at the same time, as a result of respiration.

In the daytime, therefore, when carbon assimilation is active, plants are adding to the oxygen content of the rooms in which they grow. When daylight fails and carbon assimilation ceases, respiration is still going on. Plants are then not only depriving the air of its necessary oxygen, but, more than this, they are adding to it an undesirable volume of carbon dioxide. It is for this reason that plants are removed from wards at night.

It is as a result of the intake of carbon dioxide that starch and sugar are made in green leaves.

The presence of starch in a leaf cannot be proved by dipping the leaf into iodine, because . the strong green colour of chlorophyll masks the reaction. It is necessary, first, to get rid of the chlorophyll. As this is soluble in alcohol it can be removed by boiling the leaf in. water, then leaving it in methylated spirit for some few days. The colour is extracted more quickly as a result of the preliminary boiling. A more rapid method is to boil first in water, then in methylated spirit over a small flame. If by chance the alcohol vapour ignites, it may be extinguished by putting a porcelain basin over the beaker. It is well to wash the colourless leaf in water to remove its brittleness. When it is put into iodine a brownish-blue coloration unmistakably proves the presence of starch.

It is, however, only in green leaves that starch is formed. If variegated leaves of Ivy, Geranium, or Japanese Maple are tested, starch is only found to be present in the areas that were green before decolorisation.

Starch is not made in the non-green parts of leaves, nor, evidently, does it travel to these areas from the green patches where it is made.

Certain peculiarities in the metabolism of ornamental ribbon grass, make it an unsuitable subject for this investigation.

Although it is not seen, there is abundant chlorophyll in the red leaves of Copper Beech, Red Cabbage, and Japanese Cherry. It is merely hidden from view by the red pigment, and starch is made in the tissues of all these leaves.

One condition, then, that determines carbon assimilation is the presence of chlorophyll in the manufacturing area.

Even when leaves are green, the formation of starch in their tissues is still dependent upon two other factors – one of these is light, the other is carbon dioxide.

A leaf of a Fuchsia plant, growing in a good light in the laboratory, will be found to contain much starch, especially towards the end of the day. If the plant is kept in the dark for two or three days no starch is found when the leaves are tested with iodine. The starch which was in the leaves when the plant was transferred to the dark has been turned into sugar. This has dissolved in the cell-sap and travelled away from the leaves.

As this removal of the carbohydrate from leaves goes on all through the night, and as no more is being made during the hours of darkness, a poor result is obtained on testing a leaf for starch early in the morning. Quite the best time to choose is the late afternoon of a sunny day, when the manufacture has been at its maximum.

Because of the great quantity of starch in the leaf this is not a good time to test for sugar. Nor is the early morning better, because of the escape of sugar from the leaf during the night. The best plan is to pick two or three leaves at the end of a very sunny afternoon when they contain much starch. They should be put into a corked gas-jar lined with damp blotting-paper, and kept in the dark for a few days. Sugar is then formed from the starch, but it cannot escape, and so its presence is readily detected when the leaves are tested with Fehling’s solution .

It might quite reasonably be argued that the failure to demonstrate the presence of starch, when leaves have grown in the dark, is not conclusive. Shoots are so sensitive to light that the whole plant may have suffered such a severe shock by being put into the dark that it has been unable to carry on its normal anabolic processes.

If access of light to two or three leaves only is prevented, this criticism loses its force.

Individual leaves of a Fuchsia plant growing in the laboratory, or, better still, of a Lime or other deciduous tree in the immediate neighbourhood, should be covered with black or silver paper. It is essential that the paper should fit closely, and it does the leaves no harm to pin through them here and there.

Later, when both an exposed leaf of the plant and a covered leaf are tested, the former is seen to be loaded with starch, while the latter contains no starch, except just round each pin-prick where the light has penetrated.

This marked contrast may be seen in a single leaf if designs that coincide are cut out in the middle of the paper covering both leaf surfaces. On decolorising and testing with iodine, the cut pattern is seen sharply printed in dark blue against a light background. This is what is meant by starch printing in the tissues of green leaves .

Starch, then, is found in green leaves that have grown in the light.

To prove that carbon dioxide is essential for the manufac- ture of carbohydrates, t h e access of this gas to the plant must be prevented. The plant, therefore, must be grown in a limited space so that control of the gases reaching it is possible.

It is well to use two Fuchsia plants, so that one may serve as a control. Each is put under a stoppered bell-jar fitted with a U-tube which, in the control, contains bits of pumice over which the air passes freely. The Fuchsia under this bell-jar thus grows under normal conditions.

The other plant is in an atmosphere free from carbon dioxide, the gas having been removed by a very strong solution of caustic potash contained in a dish that is put under the bell-jar along with the plant C6111206.

Undoubtedly sugar is the first product of assimilation in leaves. This may at once be carried away to various parts of the plant. On the other hand, it may lose a molecule of water and be retained, temporarily, in the leaf as starch :

C6111206 – H20 C6111005.

For its subsequent removal it must again enter into composition with water to form sugar. It can then dissolve in the cell-sap and travel to other parts of the plant, and so disembarrass the leaf of a clogging supply of starch.

For permanent storage it is carried to roots, tubers, rhizomes, corms, or seeds, or to the cortex and medullary rays of stems. In most cases a sugar solution is too bulky for permanent storage, although it is in this form that food is stored in the Carrot, and, more particularly, in the Sugar-cane and Beetroot. More usually, when sugar reaches the storage organs it loses a molecule of water and starch is formed once more. In this second appearance of starch light plays no further part, for the storage organs are shut off from the sun, being either below the earth or within the pericarp of the fruit.

In such cases starch grains are formed from the sugar in the interior of special colourless protoplasmic bodies called leucoplasts, or amidoplasts. In the cells of deep-seated organs these lie in wait, as it were, for the sugar that travels from the leaves.

Very little is known about the formation of proteins in leaves. Nitrates and sulphates absorbed in solution by the roots either reach the leaves directly or are, to some extent, decomposed as they pass up the stem.

The result in either case is that nitrogen and sulphur, in some form, reach the assimilating cells of the leaf and enter into combination with soluble carbohydrates. Possibly simple nitrogenous substances are formed and travel in a soluble form to the storage organs, where they are further synthesised, so that complex proteins result.

When the reserve food in a Potato tuber is tested the great bulk is proved to be starch. By using Millon’s reagent, or employing the nitric acid test, it is seen that the greatest quantity of protein is stored just within the skin .

By applying the various reagents, the nature of the reserve food in underground storage organs can be demonstrated.

The substances that a plant uses in the building-up of complex materials are merely, as it were, a loan. When the plant dies the products of its general decomposition are returned to earth and air. Carbon dioxide is restored to the air, ready for future use. Compounds of nitrogen and sulphur are restored to the soil, because certain soil Bacteria control a chemical reaction that is by no means understood, but which results in the re-formation of nitrates and sulphates .

Thus there is a never-ending cycle of exchange, and the substances needed for the upbuilding of plants are never wasted or destroyed.

In the tuberous root of the Dahlia the stored carbohydrate is inulin, a substance of the same chemical formula as starch. Inulin is also the reserve in the tubers of Artichokes.

Cellulose, a carbohydrate of the same numerical formula, occurs as a reserve in certain seeds .

Carbohydrates and protein do not constitute the whole of food reserves: for fats •and oils occur, more particularly, in seeds .

The changes in chemical composition leading to the accumulation of reserve food supplies do not depend upon the direct action of light.

In the work of carbon assimilation the seven rays of the solar spectrum that make up the white light of day do not play equal parts in controlling synthetic food- production.

When light passes through a glass prism it is so split up in the passage that the white light that enters the prism escapes from it as seven coloured bands : red, orange, yellow, green, blue, indigo, and violet.

When, by means of suitable apparatus, these rays are made to pass through chlorophyll, the spectrum shows certain dark bands. There is in the chlorophyll-spectrum a particularly dark band replacing the red band of the light-spectrum. This dark band is an absorption band. The red rays have been absorbed by the chlorophyll instead of being allowed to pass through it and escape.

They are trapped by the chlorophyll apparently, to supply the greater part of the energy needed in the work of photosynthesis. That red light does induce starch formation is quite simply shown by growing a plant in a large-sized box fitted with a red glass front. When its leaves are tested they are found to contain a great deal of starch. On the other hand, very little starch is found in the leaves of a similar plant grown under similar conditions, but with the red glass replaced by blue.

Indirectly, by showing that a plant makes no starch in the absence of carbon dioxide, it has been proved that leaves must take in carbon dioxide in the photosynthetic operation.

The output of oxygen during the process can be proved directly by using a water plant. A water plant is chosen because it will go on behaving normally when the whole apparatus is put under water, so that the escaping oxygen may be collected by the usual laboratory method of displacement of water.

For this purpose nothing is better than Elodea Canadensis, the Canadian Pondweed.

Bits of the plant are put into a beaker of water that stands in a cylinder of water. A funnel is inverted over the beaker and a test-tube full of water is inverted over the stem of the funnel and clamped in position. The whole apparatus should be put near the sunniest window of the room. Bubbles of gas are seen to be given off from the leaves. They rise up the water in the stem of the funnel and, passing into the test-tube, displace a small volume of water . The passage of bubbles almost ceases on a dull grey day. It is very rapid in bright sunshine when photosynthesis is active.

But, at its best, it takes quite a long time, possibly a fortnight or even more, for all the water in the test-tube to be displaced.

When the test-tube is quite full of gas a glowing splint introduced into it bursts into flame. This proves the gas to be oxygen.

To sum up : In photosynthesis, or carbon assimilation, a plant absorbs carbon dioxide from the air and returns oxygen. The process is carried on in the green parts of plants and is dependent upon light. This particular gaseous interchange, therefore, only takes place in the daytime.

Respiration, on the other hand, which involves the taking in of oxygen and the giving out of carbon dioxide, is just as active by night as by day.

To prove that in respiration a green plant gives out carbon dioxide, it must be kept in the dark. If it remains in the light the leaves take back the carbon dioxide that is exhaled and use it in their assimilatory processes.

If, however, a potted plant is put into a black box or under a bell-jar in a dark room, and if a test-tube of lime water is also put in the box or under the bell-jar, the lime water turns milky in the course of a day.

Thus half the gaseous interchange of the respiratory process, the giving out of carbon dioxide, is proved.

The other half, that is, the intake of oxygen, can be proved indirectly, as in the case of the root , by bringing about the death of the plant.

Again two Fuchsia plants can be used, each under a separate bell-jar. Under one of the bell-jars there is also a beaker of pyrogallic acid.

Just at first the plant carries on the work of photosynthesis. The oxygen it gives out in the process, as well as that originally within the bell-jar, is absorbed by the pyrogallic. There is, therefore, no available oxygen for use in breathing. Intramolecular respiration may prolong the life of the plant somewhat, but ultimately it dies from actual suffocation.

In the control experiment the plant grows healthily. Its leaves are taking in oxygen and giving out carbon dioxide in breathing. They are taking in the carbon dioxide and giving out oxygen in photosynthesis. Thus there is a perpetual cycle of interchange.

For a considerable period plants grow well under such conditions. Ferns in a glass case are generally very healthy. It is, however, necessary to admit air from time to time, because the volumes of gases taken in and given out in the two physiological processes do not exactly coincide. Also the saturated condition of the air is bad. Not only does it ultimately interfere with absorption from the soil, but it also provides an environment favourable for the intrusion of unwanted fungal growths.

In plants there is no elaborate provision for the discharge of waste, but, in addition to the carbon dioxide given off by all parts of the plant-body in respiration, surplus water is expelled through the stomata of the leaves in transpiration.

Two gas-jars, wax, and a well-fitting cork is all the apparatus needed to demonstrate transpiration.

A small branch of Lime, or other leafy plant, passes through a hole in the cork into a gas-jar containing water. In this case the waxing of all connections is to prevent evaporation of the water in the gas-jar. To make assurance doubly sure the water may be capped with a film of oil. A second and perfectly dry gas-jar is then inverted over the branch.

Drops of water collect on the sides of the upper gas-jar. This water must have come from the leaves, because evaporation from the lower gas-jar has been made impossible. It must have escaped in the form of water vapour, because it is not seen until drops condense and collect on the sides of the jar.

A modification of this experiment serves to demonstrate that the leaves give off only pure water. If a coloured solution replaces the water of the lower jar, colourless drops of water still accumulate on the glass of the upper jar.

If the apparatus is at once put into the dark, there is practically no condensation of water in the upper jar. Stomata close in the dark, therefore transpiration ceases during the night.

To see whether or no water really comes from the leaves it is only necessary to cut them off and then invert the dry gas-jar over the bare twig. It is well to vaseline the wounds first, to prevent the evaporation of water from the cuts.

As, in most leaves, stomata are much more numerous on the under than on the upper surface, it is reasonable to suppose that transpiration will take place most actively through the lower epidermis.

Accurate comparative measurements may be made by using spring balances and suspending from each a Laurel, Lime, or other fairly large leaf.

One leaf is smeared with vaseline on both surfaces. Another on the lower surface only. Another on the upper surface. A fourth leaf is left in its natural state.

The weight is carefully noticed in each case at the beginning of the experiment. The initial weight changes quickly in the case of the fourth leaf because it loses water rapidly. The loss of weight is just a little slower in the third leaf. The second has the mouths of its actively transpiring surface blocked with grease and therefore loss of weight can only take place very slowly, as a result of extremely slow transpiration from the upper surface. The first leaf, greased on both sides, keeps a constant weight.

This experiment gives accurate comparative measurements, but if spring-balances are not available, the leaves, variously greased, may just be allowed to lie on a sheet of paper or glass for several days, the changes being judged by the eye.

Naturally, external conditions play a large part in determining the rate of transpiration. A good drying-day for clothes is also a day on which transpiration is active.

The influence of a dry atmosphere in increasing the rate of transpiration is seen by putting two similar leafy twigs under separate bell-jars, whose vaselined rims rest on glass plates, so that no air enters from without.

In one case a basin of calcium chloride accom- panies the twig. The effect of calcium chloride is to remove every trace of moisture from the limited supply of air. In this dry atmosphere the leaves gradually give off all the moisture they contain. They are so dry that they can be crumbled to a fine powder.

The leaves under the other bell-jar are still fresh and green. They have given off water vapour until the air has become saturated with moisture. This point reached, they could give off no more. In the damp atmosphere, therefore, transpiration has been brought to a dead stop.

The influence of external conditions on transpiration can be accurately determined by making use of the apparatus that served to measure the rate of root-absorption . A cut branch must be substituted for the whole plant. By putting the apparatus under different conditions the influence exerted by sun, cold, wind, darkness, and humidity can be compared.

It would seem that the activity of the leaves in transpiration must cause water to travel up the stems. This does appear to be the case and may be demonstrated by putting two twigs, as nearly alike as possible, in test-tubes of equal volume containing water capped by a layer of oil, to prevent surface evaporation. The leaves are cut from one twig and the cut surfaces of the stem covered with vaseline.

The comparative rapidity with which the water level drops in the tube that holds the leafy twig is most marked. By the time the end of this twig is merely resting in a drop of oil, the other test-tube still contains almost all the original volume of water.

The leaves of the leafy twig have been instrumental in drawing water up the stem and passing it on to the outside air, so that finally they have emptied the test-tube.

It may be, then, that leaves do help to control the ascent of sap in plants. That they cannot, by any means, be indispensable factors is seen in the rapid rise of sap in trees in spring-time, when the leaves are still folded in the buds.

As the whole work of leaves depends upon their relation to the air, the stems that bear them grow in such a way as to bring them into this relation in the best possible way.

Most leaves are diageotropic, that is, their blade is parallel to the ground. It is interest- ing to notice in the leaves of creeping and climbing plants, as well as in the foliage of trees, how little overlapping there is. Each leaf is given a good chance to carry on its work. Little leaves fit into spaces between larger ones. Such a leaf-mosaic is beautifully seen in the young branches of Ampelopsis and in the Ivy. Branches of Beech and Elm also illustrate this point well .

It is possible that the individual differences in the form of foliage leaves may have arisen in response to this need for a good place in the light and air. Be that as it may, leaf modifications are manifold. One definite morphological difference in foliage leaves is that some are simple, whereas others are compound.

In a simple leaf the blade, or lamina, is in one piece.

In a Laurel or Privet leaf the edge of the blade, that is, the margin, is entire. It is an uninterrupted border outlining the shape of the leaf .

In the Beech leaf the margin waves a little. The Lime, Deadnettle, and Stinging Nettle have a saw-like, or serrate margin .

The margin of the Dandelion is dentate, being cut into prominent teeth .

In the Holly the teeth are still more prominent because they are rigid. Such a margin is spiny.

In the Oak there is more than a mere irregularity of the margin. Deep indentations cause definite lobes.

In the Chrysanthemum the lobes are more irregular and very much deeper. Deep as they are, they do not reach the midrib, and the leaf is still simple.

The blade of a simple leaf may be attached to the axis that bears it directly, without the intervention of a petiole. The leaf is then said to be sessile, or sitting upon the main stem. More commonly the leaf is provided with a petiole which determines very largely the angle made with the axis. In the angle formed between leaf and axis a bud develops.

In a compound leaf the blade is cut up as far as the midrib, so that a number of leaflets make up one leaf. All compound leaves are petiolate, and in the angle that the leaf- stalk makes with the axis a bud is developed.

Here is the proof that no matter how many leaflets there may be in the compound leaf, there is only one leaf, with an axillary bud at its base.

It has already been seen that the leaflets of the Horse Chestnut extend somewhat like the outstretched fingers of a hand. It is for this reason that the leaf is said to be palmate . Seven is a very general number for the leaflets in this case. The very large leaves may have as many as nine, and the smaller ones five or even three. Another palmately compound leaf is that of the Lupin – here the leaf is stipulate.

The commonest form of compound leaf is that in which the leaflets are disposed in pairs along the midrib of the leaf.

As there is some resemblance to the form of a feather in this arrangement, the leaf is said to be pinnate, from the Latin pinna, a feather.

The flaming Sumac tree has pinnately compound leaves, and so Pinnate. Has the Ash .

In the latter the corn-pound leaves grow on the twig in alternate pairs, but the members of a pair are not exactly opposite one another. The young part of the stem that bears the foliage leaves is curiously flattened. The first pair of axillary buds is very close to the terminal bud and all the buds are black. It is just possible that the tree owes its name to the sharp contrast that the ashen grey stem makes with the coal-black buds that grow upon it.

The pinnately compound leaf of the Rose has two stipules at its base which are adnate – they are attached for the whole of their length to the leaf-stalk. Small decurved hooks occur on the under surface of the leaf, both on its midrib and on the main vein of each leaflet.

In the stipulate and pinnately compound leaf of the Sweet Pea all but the lowest pair of leaflets are modified to form tendrils .

When a compound leaf is made up of three leaflets it is ternate. Laburnum, Strawberry, Blackberry, Wood Sorrel, and Clover all have temately compound leaves .

The sleep movements of the temate leaves of many members of the family Leguminosx, and of the Wood Sorrel, prevent transpiration from taking place too actively when loss of water would be a disadvantage to the plant, as in the cool of the evening or in great heat.

In the White Clover the stomata are most numerous on the upper surfaces of the leaflets, therefore it is these surfaces that the sleep movement is designed to shield.

The two lateral leaves droop forward in such a way that their upper surfaces meet. The terminal leaflet then bends over them, roofing them in as it were .

The sleep movement is different in the Wood Sorrel, because here the stomata are chiefly on the lower surfaces.

As night draws on a downward and backward droop of the leaflets brings the midrib of the underside of each leaflet up against the leaf-stalk. Each leaflet then bends along its midrib so that one half of the under surface of each leaflet, where the stomata are numer- ous, is in close, flat contact with one half of the under surface of its neighbour .

A little paper-cutting and -folding will make the procedure in these two cases very plain.

In Clover and Wood Sorrel the tropism is a response to the stimulus of light. The mechanism of such movement in leaves is by no means understood. It is apparently controlled by the pukinus, a somewhat swollen basal portion of the leafstalk. It may be that a loss of turgescence in one half of the pulvinus causes the fall of the leaflets in the sleep movements. A recovery of turgescence in this region and a loss of turgescence in the other half of the pulvinus would then bring the whole leaf back to its normal, outspread diurnal position.

The movements of these leaves are periodic, occurring as regularly as the alternation of day and night. Similar move- ments in the leaves of the Sensitive Plant are induced by contact.

A response and a mechanism similar to those of the Clover and the Wood Sorrel must determine the movement of a leaf during its growth period, the ultimate result being that its petiole has formed such an angle with the stem that bears it, that it is rightly placed with regard to the direction and intensity of light.

Hence the formation of leaf-mosaics.

In some very specialised cases leaf movement occurs in connection with feeding, when the plant supplements its normal inorganic food supply by absorbing the soft tissues of animal organisms.

In Britain there are only three genera of Carni- vorous (Insectivorous) Plants – Pinguicula, the Butterwort – Drosera, the Sundew – and Utricularia, the Bladderwort.

In all there is provision for the digestion and absorption of certain animal tissues, and it is always the leaf that functions as the trap.

Then again, all Carnivorous Plants have this in common : they grow in habitats markedly deficient in nitrates. It would seem that the carnivorous habit has arisen in response to the need for a supply of nitrogenous food, to make up for the lack in the soil in which they are rooted.

The Butterwort grows on boggy, mountain slopes in various parts of Britain, on the hills of the Highlands and Lowlands of Scotland, in Wales and in the Lake District.

The formation of nitrates in soils depends very largely upon the action of Bacteria . In boggy areas there is so little available oxygen that bacterial activity is hindered. It is for this reason that the soil suffers a nitrate-poverty.

The Butterwort, growing in such a habitat, has its somewhat trough-like foliage leaves arranged in the form of a rosette, in the centre of which rises a delicate stalk, bearing a single flower which is of a bluish-violet colour .

The edge of each foliage leaf is just slightly upturned and its surface is rather sticky, owing to the secretion of a mucil- aginous substance from numerous glands dotted all over it. It is estimated that there are about 25,000 of these glands on one square centimetre of the leaf.

When small flies alight on the leaf it becomes still more trough-like. Its edges fold slowly inwards and the captives are pushed more and more towards the middle of the leaf. At the same time the mucilage is not only flowing more freely, but its character has changed. It now shows a definite acid reaction and digests the protein with which it comes in contact. The mucilage is, in fact, similar to the gastric secretions of the animal stomach –

It is interesting to note that the actual digestive juice cannot be induced to flow by mere contact. A grain of sand, for instancy, produces no result.

If, however, a minute piece of lean meat, or of cheese, is dropped upon a Butterwort leaf the discharge of mucilage increases in volume and its character slowly changes. When the process of digestion is complete the excess of mucilage and acid is reabsorbed by glands of the leaf, along with the digested food.

The Sundew is found in the same habitat as the Butterwort, and in the Northern uplands the plants often grow side by side. Sundew has, however, a wider distribution, and is found as far south in England as the Surrey heights.

The plant takes its name from glistening red tentacles that project from the upper surfaces of the foliage leaves, which are arranged in a basal rosette.

The slender tentacles are club-shaped at the free end, where a glistening, honey-like drop of mucilage is secreted. It is these dew-like drops that attract the prey to the leaves .

The capture is effected by a slow inward bending of the tentacles, one after another, in a clock-wise direction. Contact with an inorganic substance induces a slight response in some of the tentacles, but the movement is by no means general, and the responding tentacles quickly right themselves. The contact of an insect, or of the smallest particle of any organic substance, causes not only the movement of the tentacles but, at the same time, a free flow of glistening liquid which contains an enzyme and a weak acid. To hold the captive and the digestive secretions the leaf itself generally becomes slightly concave.

The digestive fluid acts upon the organic substance and the products of digestion are absorbed by the leaf. The hard parts of an insect, the wings, legs, eyes, and stomach, remain on the leaf-surface. When the process of absorption is complete the leaf is found to be quite dry. The advantage of this is that the hard, undigested parts lie loose on the leaf-surface, and are washed away by rain and blown away by wind. Freed from their hampering presence, the field is prepared for further action. Once more the tentacles exude their glistening drops to lure other victims to their doom.

The Bladderwort is a submerged, rootless water plant, with finely divided leaves which carry on the work of photo- synthesis, making use of the carbon dioxide dis- solved in the water in which the plant grows.

Certain parts of the leaves are curiously modified to form bladders, and it is their work to supply the necessary nitrogenous compounds .

The opening into the bladder is guarded by a flap suspended from the upper border. Its lower edge rests on a small pad of tissue in such a way that the flap responds at once to pressure from without, whereas pressure from within closes it all the more firmly. Projecting from the neighbourhood of the mouth there are stiff, branching hairs.

Cyclops, or Daphnia, possibly fleeing from a young newt, make for safety among the hairs. Slight pressure on the valve gives them an entry into the bladder. They have escaped death in one form only to meet it in another. The bladder is lined with glands whose secretions digest the captive. Ultimately the products of digestion are absorbed by the tissues of the leaf.

Dimorphic leaves, that is, leaves of two forms, occur in another well-known water plant, the Water Crow- foot , which covers the entire surface of many ponds in early summer. The whole spread of the plant is very decorative and starry, for the white petals are golden at the base and the flowers stand up against a background of shining floating leaves. These floating leaves have a cuticularised epidermis on the upper surface to prevent excessive transpiration. The submerged leaves are very finely divided. This modification is a great advantage to a plant growing in stagnant water. The slightest ripple in the pond changes their position, so that they are brought into contact with a different volume of water. This gives them a fresh source of supply of carbon dioxide for photosynthesis and of oxygen for breathing. . Leaf movements in Clover are made in response to light. In the Sundew they are induced by con- tact of a special kind. In some xerophytes leaf movement is determined by the hygrometric condition of the atmosphere. If the air is dry the leaf folds in some way. In a moist atmosphere it expands.

As its name implies, a xerophyte is a plant that grows in a dry situation (Greek xeros, dry – phyton, a plant).

The disadvantages under which xerophytes labour are two-fold. In the first place they grow in situations where there is such scarcity of water that they have difficulty in absorbing enough for their needs. In the second place, as they are without shelter, exposed to the full force of sun and wind, there is every inducement for them to part with the water which they have, with so much difficulty, secured.

The disadvantage as regards absorption is got over to some extent by the possession of unusually long tap-roots. The tap-roots of Thistles are often as much as, or even more than, six feet in length.

In some cases, too, the plant is its own storehouse of water, conserving in its leaves or stems a water supply that makes the scarcity in the soil of less importance.

In House-leeks and Stonecrops that grow on rocks and walls, water is stored in the leaves. On pressure a drop of water is readily squirted from a fresh Stone- crop leaf. The leaves of the Samphire of our cliffs are succulent for the same reason. So, too, are the much larger leaves of Mesembryanthemum, the Ice Plant, which, although it is not British, grows so freely by the sea in the south of England.

In the numerous Cactus varieties, water is stored in the stems.

The contrivances to ensure reduced transpiration in xerophytes are manifold.

As water is given off from the foliage leaves it is obvious that reduction of these would be a ready means of limiting the rate of transpiration.

The most drastic reduction would be to dispense with foliage leaves altogether and let some other part of the plant concern itself with photosynthesis.

This is exactly what has happened in the Butcher’s Broom , where the leaf-like axillary dwarf branches have taken on the work of assimilation. In the Cactus, stems of various shapes manufacture the carbohydrates and proteins, and bear only scale-leaves that are very much reduced.

A leaf that is horizontally extended will, other things being equal, give off much more water than one that exposes only a small surface to the light and air. It is therefore an advantage for the foliage leaves of a xerophytic plant to depart from the typical laminate shape.

Thus in the Gorse the leaf is short and cylindrical in shape . An accessory modification is found in the assimilating ridges of the stem. Similar ridges occur, too, in the Broom, where the foliage leaves are much reduced in size .

In the Pine, although the foliage leaf is long, it is very narrow and thick . Its epidermis is cuticularised as an added protection against undue transpiration, and the stomata, through which the leaves transpire, are sunk below the level of the general epidermis. The cells that flank each stoma arch over it slightly, forming a minute vestibule into which water vapour from a stoma escapes. Thus the air in this very limited area is saturated with moisture, with the result that further transpiration from the leaf is delayed to some extent.

In Heather not only are the foliage leaves very small and closely packed, but the blade of the leaf is rolled so as to enclose a hollow chamber . It is in the epidermis lining this chamber that the stomata occur. The stomata occupy a similar position in the Crowberry and other heath and moorland plants.

In all these cases the reduction in the size of leaves is permanent. A temporary reduction, determined by external conditions, takes place in some cases, and is due to 4 – Temporary the folding of the leaf.

The grass that covers the sand-dunes of our coasts is a good example of such folding. This grass, the Sea Marram, or Psamma, has a very long blade which is grooved lengthwise. Further defence against undue transpiration is provided for lie. In wet weather the grass blade is flat and strap-like.

This, commonly called bloom, is seen middle line of the leaf, forming a hollow cylinder. It folds in such a way that the stomata, which are on the upper surface of the leaf, are within the cylinder, and hence protected from sun and wind . It is when the leaves are in this rolled condition that, in walking over the dunes, one’s legs are constantly pricked by the sharp tips of the rolled blades. At its best in the outermost layer of the fruit coat

Such leaf-rolling is characteristic of many grasses. It depends upon the presence of hinge-cells at the bases of the grooves. These regulate the in-folding of the leaf by losing their turgidity. In regaining turgidity they cause the blade to flatten out.

It is a similar response to conditions of moisture that makes the Rose of Jericho an object of interest. Its branches, which are consistently curled inwards, change their position and bend outwards when the plant is put into a saucer of water.

Later the outer walls of the epidermal cells are cuticularised, that is, a layer of cutin, in many respects resembling cork, forms over the cellulose wall. Such cuticularisation hinders transpiration from taking place through the whole epidermis and helps to limit it to the stomata only.

In many xerophytic plants the cuticle is developed in a high degree. The shiny leaves of evergreens owe their characteristic appearance to cutin. Under surface where the numerous stomata occur. The blocking of the stomata by still air entangled in the hairs, and by air saturated with water vapour, appreciably diminishes transpiration.

Other notably hairy leaves are those of Mullein, which grows in dry, waste places, and the very soft and silvery-looking leaves of the Stachys of the garden, called Pussy’s Ears by young children.

Very frequently the actual arrangement of foliage leaves is a bar to rapid transpiration. This is true of radical leaves, that grow close to the ground in the form of a 3 rosette .

The upper leaves of the rosette of a Dandelion, growing by the roadside, are often seen to be withered, while those below them are healthy and turgid.

The upper leaves have been sacrificed for the general good.

The shelter they have given has prevented excessive transpiration from the lower leaves of the rosette. The grooving of the individual leaves, as well as their arrangement, directs rain and dew to the region where the long tap-root is growing vertically downwards. Also, because of its contact with the soil, the rosette prevents some evaporation from the particular area it covers, which is, in consequence, not quite so dry as the rest of the ground.

The leaves grow from a short vertical rhizome above the tap-root. The reason why the rosette does not rise higher and higher above the ground level as the years pass is, that the branch roots of the tap-root, like the adventitious roots of rhizome, bulb, and corm, are contractile and, by their slight shortening, are able to maintain a constant relation between rosette and soil .

In the leaf rosettes of many Thistles a still further protection against rapid transpiration is provided by a weft of delicate hairs, often superficially resembling a spider’s web, which occurs in the centre of the rosette, more particularly when the leaves are young.

Experiments carried out on root-absorption showed that the power of roots to take up water diminished as the temperature dropped . In Britain root-absorption ceases in the cold winter soil. If the leaves were actively transpiring at this time they would be losing water at a rate that would spell disaster. It is to prevent this that the leaves of evergreens, Laurel, Holly, Pine, have such a strongly cuticularised epidermis.

Deciduous trees avert catastrophe by following a drastic course, and before winter is actually upon them they ruthlessly cast off all their leaves.

The fall is by no means haphazard.

Certain cells at the base of the leaf-stalk become separated one from another, and so form an area of weakness. It is here that the leaf eventually breaks away. The veins of the leaf, continued in the petiole as vascular bundles, do not take part in this disintegration.

It often happens, therefore, that the leaf hangs from the twig for a time supported only by vascular bundles and, possibly, by the epidermis. The dividing layer, which is called the absciss layer, occurs also at the base of each leaflet of most compound leaves.

Either while the cells of the absciss layer are separating, or immediately afterwards, a formation of cork in this area heals what would otherwise be an open wound. Before the cork formation takes place there is an interchange of substances between the leaf and the branch that bears it. Any manufactured food that is still in the leaf passes along the bast of the veins to the leaf-stalk, and so to the branch where it is stored, probably just underneath the bark, in close proximity to the resting buds. The buds are able to draw on this reserve supply when they open in the spring-time.

Trees also make use of leaf-fall to get rid of waste materials. In plants, except for the giving off of carbon dioxide and water, there is no regular discharge of the waste products of metabolism. Unwanted substances do, however, pass into deciduous leaves before they fall.

If a branch is blown off, or hangs broken from a tree, its leaves wither and curl up, but they do not drop. This makes it clear that leaf-fall is a result of life, not death. It is the living protoplasm of the cells that controls the formation of the absciss layer and the deposition of the protective covering of cork.

By way of preparation, then, for winter, trees cast off their leaves, the fall being preceded by a safeguarding of food.

Further, at the same time that a cork layer is forming over the leaf-scar, the lenticels in every part of the trce are being closed by a similar protective layer. Just as in any other hibernating individual, the dormouse, the hedgehog, or chrysalis, so in trees, the respiration will be at a minimum during the rest period of winter. In spring the thin cork layer of the lenticels crumbles, and they are functional once more.

In herbaceous perennials the preparation for winter, or for a seasonal rest period, consists in the complete dying-off of all parts that grow above the ground. Before this happens food travels from the leaves to be stored in the underground perennating organ, whether this be root, rhizome, bulb, or corm.

In annuals, as the plant dies completely, the only hibernation possible is in the form of seeds.

Biennials follow the herbaceous perennial plan at the end of their first year of life but, like annuals, stake their all on seed-formation at the completion of their second season.

In trees the beautiful autumn colouring that precedes the fall of the leaf is more easily seen because of the decom- position of chlorophyll and its ultimate disappear- ance. As a result of this the colours of other pigments, formed at this time, are plainly seen.

The most vivid colours occur when sunny autumn days are followed by frosts. The abrupt change from summer to winter conditions brings the work of the leaves to a sudden end.

In the sunshine, photosynthesis was active and sugar was produced in the leaves. The sharp night frosts put an end to this metabolic process, but the leaves retained their sugar. There is some connection between the presence of sugar in leaves and their red coloration.

The Sugar Maples of Canada have flaming autumn foliage. These leaves are very rich in sugar and, as they grow in a climate where warm and sunny days are followed by sharp frosty nights, all the conditions are perfect for producing the glorious colourings of the vivid Canadian fall.

Contradictory though it seems, plants that grow in water or in damp marshy places, show many of the adaptations that occur in xerophytes for the reduction of transpira- tion.

The sword-shaped leaves of the Iris are grooved and the sunken stomata are sheltered in the grooves. The leaves of the Sweet Gale are very small. The floating leaves of the Water Lily and Water Crowfoot are cuticularised. The leaves of the Osier Willow are silvery-white because of the hairs on their lower surfaces – these surfaces in some of the less well-known Willows are coated with a deposition of wax. The Mealy Primrose of the moss lands of the north of England and south of Scotland takes its name from the unique nature of the leaves – the white floury appearance of their lower surfaces is due to wax deposited in small rods or granules to protect the stomata – the leaves, too, grow in the form of a rosette.

Such adaptations, obviously intended to regulate the output of water, indicate that even in water plants absorption is none too easy.

The fact is that damp earth is cold earth. In determining variations in the rate of root absorption , there was seen to be definite loss of activity when the gas-jar was packed round with ice. Roots in a cold environment absorb slowly, and it is partly for this reason that precautions are taken to prevent excessive transpiration from the leaves.

Not only is damp earth cold, but it is also poor in oxygen. The roots are not breathing freely, and this is another hindrance to active absorption. Then, again, when boggy land dries in time of drought it becomes unusually hard, dry earth. At such seasons the xerophytic adaptations of water plants are a great safeguard against reduction of the plant’s water content.

In salt marshes, too, the ground is cold and poorly supplied with oxygen. More than this, substances in solu- tion in the soil-water now hinder absorption. In the experiment just referred to it was proved that the addition of salt to the water in the gas- jar caused a great retardation of root- absorption, because the normal course of osmosis was interfered with.

Thus it is not surprising to find that plants growing in such situations also show marked xerophytic adaptations.

The small leaves of the Saltwort are covered with fine down. In the very succulent Glasswort the leaves are mere scales . The mealy appearance of the leaves in the Common Orache is due to protecting wax.

It has been suggested that in halophytes the stomata lose the power of closing. The water given off in transpiration is pure water – substances in solution are left behind in the leaf. It may be, then, that in salt-marsh plants, small depositions of salt block the stomata, so that they cannot close.

If transpiration were very active in these plants the sap of the cells would, of necessity, become more and more dense, and the whole economy of the plant would thus be thrown out of gear.

The buoyancy of water plants depends upon their numerous, and often large, internal air-spaces. As well as making for lightness, the spaces store up air, in which oxygen and nitrogen are practic- ally in the same proportion as in the atmosphere. It appears that the plant organs draw upon this supply in respiration, finding it easier to use the oxygen of their own store than that dissolved in the water. The air-spaces are very regularly disposed in root and stem, as may be seen by cutting across the axis of the Mare’s Tail and looking at it with a lens .

Another characteristic of the internal structure of water plants is the great diminution in the number of wood-vessels. In many submerged plants, as in the Grass Wrack, they are entirely absent. As water is taken in by all parts of the plant, well-regulated conduction is not necessary. In the Mare’s Tail there is a need for wood-vessels, because the long stem projects far above the surface of the water, and solutions must be carried upwards to the leaves.

In such cases, although they are present, the number of wood-vessels is very much reduced.

Many floating water plants, in hibernating, retreat bodily to the floor of the pond in which they are growing. No Water Crowfoot is seen on the surface of the water in autumn.

In other individuals, while the great bulk of the plant dies away, definite buds are separated off, drop to the bed of the pond, and lie dormant until the following spring . Buds are detached in this way in the Floating Pondweed and the Frogbit. Before they separate, food passes to the buds for storage. Sugar changes to starch, and it is the weight of the accumulated starch grains that makes the buds heavier than water.

In the spring there is a reverse change of starch to sugar. The tightly packed bud expands and grows and, in its growth, uses much of the reserve supply of food. As a result of this reduction in the stored carbohydrate, together with the expansion of the cells, the whole bud becomes so light that it rises to the surface of the water. The axis of the bud then elongates – axillary buds develop into branches, and the individual enters upon a new period of life.

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