THEwhich, in one way or another, reaches the ground to germinate, has a protective coat, or testa. Within this is the embryo, or rudimentary plant. A reserve food supply is absolutely essential for the use of the embryo when first it begins to grow. In some this store is contained in the embryo itself. In others it occupies an independent and is then known as endosperm.
The Broad Bean seed is so big and so cheap, and thedoes all that is required of it in such an exemplary fashion, that we naturally turn to it for preliminary study.
The seed is made up of two parts. The first of these is the testa. The second is all that is within the testa and is, in this case, the embryo, and the embryo only.
The testa is of a uniformly drap colour, broken by a sharply defined, long black band at one end. This mark is the hilum and denotes the region where first the ovule, and later the seed, was attached to the funicle, the boat-shaped stalk that formed the attachment to the pod .
On the curved rim of the seed, above one end of the hilum, a triangular bump marks theof the radicle. Between the apex of the bump and the end of the hilum there is a minute hole, or micropyle, through which the pollen-tube entered the ovule in fertilisation. The presence of this hole is made obvious by soaking the seed in water for forty-eight hours. Gentle pressure near the hilum will now cause drops of water to exude from the micropyle. It is obvious that the hole is well placed in relation to the tip of the radicle that must, sooner or later, emerge.
In such soakedit is an easy matter to remove the testa whole, when its tough character is evident. On one curve there is an inner pocket into which the radicle fits. The radicle is in a somewhat unprotected position because of being so near the micropyle, which is an area of weakness, and this pocket gives it extra protection.
When the testa is remoyed the naked embryo is fully exposed. It consists of three parts : (1) Two seed-, or cotyledons, which are attached laterally to a very short primary axis. (2) The radicle, or young , which points directly downwards from the primary axis. (3) The p/umu/e, or rudimentary bud, which grows directly upwards from the primary axis. It lies between the cotyledons at right angles to the radicle. The plumule develops into the plant’s whole system of -bearing, flower-bearing, and fruit-bearing parts.
Clearly theis not an upward continuation of the root, any more than the root is a downward prolongation of the stem. In the early cell divisions which follow the fertilisation of the egg, certain cells are marked off to become the plumule, or shoot, while others are destined to form the root.
Testing with iodine shows that the cotyledons contain large quantities of starch. In this test it is interesting to scrape one cotyledon and leave the other intact, in order to see the resistance provided by the protective epidermis.
To prove the presence of protein the cotyledons should be well crushed with water.
One portion turns red when tested with Millon’s reagent and boiled .
Another portion should be subjected to the nitric-acid test. When the crushed mass is treated with concentrated nitric acid it turns yellow on warming. When the acid is replaced with ammonia a bright orange colour results.
The Biuret reaction gives a very pretty test for proteids. If the remaining portion of the crushed cotyledons is treated, first with I c.c. Of 40 per cent. Caustic soda solution, then with a drop of I per cent. Copper sulphate solution, a violet colour is produced.
There is, then, ample provision in the cotyledons for the needs of the seedling. It only remains for the stores of starch and proteins to be made available for absorption by the root and shoot. Given a suitable temperature and a supply of water, cells of the embryo secrete enzymes that bring the stored foods into this condition. The work of one class of enzymes is to break down the cellulose walls of the cells of the cotyledons and so prepare the way for the enzyme diastase which changes the starch into sugar. The proteins, in their turn, are also brought into soluble form.
In germination the pressure of the growing root tears the testa at the point of weakness and its tip emerges in the region that was once the micropyle .
When the root is well established in the ground, t h e plumule grows up. Its tip is bent like a crochet-hook, so that the delicateof this first bud are not subjected to friction . Once the tip has broken through the surface of the soil it becomes erect by the straightening of the stem.
The first leaves borne on the stem are simply stipules . These are succeeded by compound leaves made up of two leaflets and a rudimentary tendril. At the base of each compoundis a pair of stipules. The later leaves have three pairs of leaflets and the rudimentary tendril is still present.
The elongation of the stem of the plumule, the primary bud of the plant, like the corre- sponding elongation of the axis of a resting-bud, separates the nodes widely. The leaves, which were originally crowded together, are now separated one from another by long inter- nodes, and the only crowding is that of the young leaves, con- tinually being formed from the growing point .
It is at this stage of growth that doubt as to the morpho- logical nature of the cotyledons disappears. They look remarkably un-leaflike as they lie within the testa. Now, if the testa: is removed and the cotyledons opened out, a bud is seen in the angle that each cotyledon makes with the axis. The presence of these axillary buds proves, beyond doubt, that the cotyledons are really leaves. If the main shoot is injured in germination one, or both, of the buds develops in its stead.
There is a difference in internal structure between root and. The, area in which the change from one to the other takes place is called the hypocotyl. In the Broad Bean this area, below the cotyledons, is extremely short .
Even when the root has penetrated the soil the reserves in the cotyledons are still being used as food. The seedling has, in fact, no other supply, for it is not until the leaves are well expanded that the new plant can carry on the work of.
A proof that the cotyledons do supply the food is gained by very carefullythem away from a seedling, leaving the elongating radicle and plumule in the moist air of the gas-jar. There is no further development, and both root and shoot die.
The same truth may be demonstrated by a comparison of weights.
A completely dry seed must be weighed before it is soaked in distilled water. After soaking it is allowed to germinate in a gas-jar with blotting-paper and distilled water, the growth continuing until both root and shoot are several inches in length. These are then most carefully cut off quite close to the testa. The testa and what remains of the cotyledons within it are then allowed to become just as dry as was the original seed before soaking. In this state it is weighed, and this second weight is found to be appreciably less than the first, showing that food from the cotyledons has been used in the building up of root and stem. Of course, in weighing the original seed the weight of radicle and plumule is included – but even when allowance is made for these very minute structures the difference in weight is marked.
Actually these somewhat elaborate experiments are not necessary to prove the point. Seedlings in gas-jars in the dark grow greatly. Their leaves cannot make food in the dark. There is nothing in the way of nutrition to be got from distilled water. If the food necessary for growth was not provided by the cotyledons, what other possible source was there ?
It is only common sense to suppose that seeds will not germinate unless they are provided with air, moisture, and a certain degree of temperature. It is easy to prove that this is really so by depriving seeds in turn of these conditions and noting the result. Barley,
Oats, Cress, and Mustard seeds may all be experimented with in this way.
Instead of using blotting-paper, a layer of cotton-wool is put at the bottom of four clean gas-jars. In one dry seeds are sprinkled on the dry cotton-wool and the jar is kept in an ordinary warm place. The seeds have plenty of air, a moderate temperature, but no moisture. The result is that germination does not take place.
In the remaining gas-jars soaked seeds lie upon damp cotton-wool.
Water that has been boiled to expel air, and then allowed to cool, is added to one set of seeds, until the jar is a little less than half full. A layer of oil on the top prevents air from coming into contact with the water from without. In this case the seeds have plenty of water, but no air.
Again they do not germinate.
The third gas-jar stands in a pneumatic trough, packed round with ice. Air. And moisture are both available, but the cold prevents germination.
The fourth jar stands on a bench in the laboratory and the seeds germinate. They are provided with air and moisture and are at the ordinary room-temperature.
The need for air depends upon the fact that every living thing must breathe. Respiration experiments on mature plants have to be conducted in the dark because, in the light, their green leaves give out oxygen and use up carbon dioxide in the work of photosynthesis. Experiments relating to the breathing of seeds can be carried out openly in the laboratory, because there are no green leaves, in this case, to complicate the gaseous interchange.
As respiration includes both the giving out of carbon dioxide and the taking in of oxygen, the experiments must be conducted with both these processes in mind.
Soaked peas are eminently satisfactory for use in these demonstrations. In the first experiment they should be put into a flask, on the bottom of which are two or three layers of damp filter paper. A test-tube of lime-water is let down into the flask by a cotton thread and the flask is corked . The next day the lime-water is milky, just as if it had been breathed into by ourselves. The milkiness is fine particles of chalk suspended in the water, because the following chemical reaction has taken place :
CO2 Ca (OH)2 – ›- CaCO3 + H20. Caustic soda chalk An additional proof that the peas have given out carbon dioxide is gained by putting a burning taper into the flask. The flame is extinguished the moment it enters the neck of the flask. The peas have given out carbon dioxide – the air in the flask no longer supports combustion and the lime-water has become milky.
The extinguishing of the taper suggests that the oxygen originally present in the flask has been used up. To prove that this is so a flask and soaked peas are arranged as before, but a test-tube of strong caustic potash solution takes the place of lime-water. The cork is replaced by a one-holed rubber stopper, through which passes a glass tube bent at two right angles. The long vertical arm of the tube dips into a beaker of some coloured solution.
Along with the peas in the flask there are, originally, nitrogen, oxygen, and a little carbon dioxide. The original carbon dioxide is absorbed by the potash. Assuming that the peas do breathe they will use the oxygen in the flask and give out carbon dioxide. But this will at once be absorbed by the potash, with the result that the volume of gases in the flask is reduced. This tendency to form a partial vacuum is counter-balanced by the air in the bent glass tube entering the flask. But naturally the drawing in of the air is followed by the drawing up of the coloured solution, which rises rapidly in the long vertical arm of the tube, to take the place of the oxygen used by the peas.
Hence : germinating seeds take up oxygen and give out carbon dioxide. That is, they breathe.
Intramolecular respiration of plants has already been referred to . It can be demonstrated by using peas. A very small test-tube is necessary or, better still, one of the small specimen tubes that the dealers use when they supply Ameba and Chlamydomonas.
The tube, filled with mercury, is inverted in a small dish of mercury and clamped vertically. Four or five soaked peas have their testas removed and are then inserted, one by one, into the mouth of the tube. The light seeds rise quickly in the dense mercury to occupy positions at the upper end of the tube. There is no gas of any kind in the inverted tube, because it was first of all completely filled with mercury. No air has been admitted with the peas because the precaution was taken of removing their testas, lest there should be a little air between these and the embryos.
As the days pass the mercury sinks gradually lower in the inverted tube and the peas are now seen resting upon it, occupying a clear space in the tube . The sinking continues until the whole of the mercury has left the tube. It is plain that something must have forcibly pushed out the heavy mercury – that this invisible something must be a gas – that it must have come from the peas, because there is absolutely nothing else present that could have produced it.
The tests for this gas must be con- ducted very carefully – there is so little present in the very small tube that one cannot afford to waste any.
Carbon dioxide is heavier than air. In removing the tube from the clamp, therefore, the thumb must securely cover its mouth and the tube must be inverted as quickly as possible. There is no room for a test-tube of lime-water this time, so a drop of clear lime-water, hanging from the end of a glass rod, must be brought to the mouth of the tube. It becomes quite definitely milky. When a burning match is put into the tube the flame is at once extinguished.
There is, then, no room for doubt. The peas have given out carbon dioxide – they have carried out one half of the breathing process. As they were supplied with no oxygen from without, the only possible source from which they could derive it was their own tissues. It is this process, in which plants use the oxygen actually within themselves in breathing, that is known as intramolecular respiration.
The degree of temperature below which seeds will not germinate is, naturally, not constant. The seeds of tropical plants will not germinate unless they have much more warmth than is required by peas and beans and hardy Northern plants generally. There is, in fact, for all plants a certain optimum temperature, at which germination most readily takes place.
Temperature and moisture together give the stimulus for the secretion of the enzymes that are essential for changing the reserve foods into an absorbable form. Water is taken in all over the testa and the first effect of absorption is to cause the seed to swell. This is really swelling, not growth. Growth is a permanent change of form, but if the swollen seed be allowed to dry it shrinks, gradually, to its former size. The pressure exercised by swelling seeds is known as the force of imbibition, and is surprisingly great. It is only necessary to put some dry peas, with water, in a test-tube to realise the truth of this. As the seeds swell the test-tubes are splintered. The use of swelling peas in skull-dissection has already been indicated .
In the germination of a Broad Bean the cotyledons always remain below the ground. This is known as a hypogeal condition. It is not nearly so usual as the epigeal type of germination, in which the cotyledons come up above the soil-level as the first green leaves of the plant. When the seeds that we have sown in the garden come up it is the paired cotyledons that are seen making the green pattern above the soil.
In the French or Kidney Bean the cotyledons are epigeal. The structure of the seed is similar to that of the Broad Bean in all essentials, but there are differences of detail.
The plumule is perhaps a little more feather-like and the radicle and plumule occupy a different position, so that the micropyle is also differently placed .
In the germination of the Kidney Bean it is a simple matter to determine the value of the cotyledons, for when they are well up above the coco-nut fibre they can be cut off from two or three plants. These plants invariably die, being deprived of their food supply, while the others grow sturdily for some time.
As the plumule in the seed lies between the cotyledons, it is between the cotyledons that the shoot must be looked for in the seedling. The long, elongating axis cannot, therefore, be any part of the shoot. It is, in fact, not stem at all, but the elongated hypocotyl . In all cases of epigeal germination the hypocotyl elongates in this way and the plumule unfolds between the two protecting cotyledons. Before they drop off, the cotyledons, having become green in the light, to some extent carry on the work of photosynthesis .
When a bean seedling grows in distilled water in the light its weight increases, partly because of the starch and sugar made in photosynthesis, but largely because of the water thehave absorbed.
To determine the actual solid, or dry weight, the seedling should be carefully dried in an oven until all water is driven off.
To determine the gain in dry weight during germination comparison must then be made with the dry weight of an ungerminated seed, similarly heated.
Comparison may also be made with a seedling grown in the dark. In this case no photosynthesis has taken place. The wet weight of the seedling will, of course, be greater than that of the seed, because water has been absorbed. The dry weight of this seedling will, however, be less than the dry weight of the seed, because the reserves in the seed have been broken down during the growth of the seedling and carbon dioxide has been given off.
When a potato tuber sprouts in the dark, lying on the floor or on a shelf, it loses both dry weight and wet weight fairly rapidly, for it is giving off both carbon dioxide and water. Its branches may cover quite a wide area, but, none the less, tuber and branches weigh considerably less than the unsprouted potato.
Similar distinctions between dry weight and wet weight may be noted in foliage leaves.
By the end of a sunny day a leaf has given off a good deal of water, but it has made a great deal of starch. Its dry weight is therefore considerable.
A similar leaf in the early morning may weigh as much actually, because it has not been transpiring during the night. But if it is dried and then weighed, its dry weight is found to be less than that of the leaf picked in the late afternoon, because its starch has been converted into sugar and has travelled away from the leaf during the night.
What florists sell asseeds are, in reality, Sunflower fruits. They are the cypselas of ‘the disk florets of the capitulum of the Sunflower. They can be seen, beautifully arranged, on the head of any Giant Sunflower, when the flowering-time is over.
The brittle pericarp is often somewhat striped, white upon brown. The scar at its pointed end marks its attachment to the receptacle. Another small scar in the notch at the farther end is the point where petals and style have withered away .
There is so much air between pericarp and seed that the fruits always rise to the top of the water when they are being soaked.
The seed itself is covered with a testa so delicate that neither hilum nor micropyle can be seen, even with a hand lens. When the testa is removed and pressure released, the two vertical cotyledons spring apart and disclose the minute plumule which lies between them. The radicle is peg-like and points downwards .
In this case, as in beans, the cotyledons store the food required in germination. It is, however, in the form of oil. If broken cotyledons are heated on white paper in a porcelain basin an oily smear spreads along the paper. On the addition of a drop of osmic acid a black stain confirms the presence of oil.
The germination of the Sunflower is epigeal, like that of the Kidney Bean. Both testa and pericarp protect the cotyledons when the elongation of the hypocotyl pushes them up through the soil. Several days pass before the cotyledons expand and the coats are thrown off .
It is interesting to examine as many seeds as possible and note their differences. In the Sycamore, for instance, the cotyledons are very long and are tightly coiled. As they have always been within the dark fruit, shielded from the light, it is a surprise to find that they are green. The cotyledons of the Scotch Fir show this same unusual characteristic.
The seed of the Ash differs from all the foregoing in that the embryo does not fill up all the space within the testa.
In a soaked Ash samara the brown seed is found lying attached by its long, curved funicle to a placenta at the upper end of the fruit. To see the embryo, the solid part of the seed must be sliced carefully with a sharp scalpel. An almost indistinguishable plumule lies between the two delicate cotyledons, and the apex of the peg-like root is directed away from the fruit stalk .
Such delicate cotyledons cannot hold any food reserve. In this case the seedling’s needs are provided by the endosperm, the tissue in which the embryo lies. In the Ash the endosperm is very largely cellulose, as it is in the Date. The test for cellulose is more easily carried out in the Date than in the Ash, because it is so much bigger. When the broken Date stone is treated first with iodine, then with concentrated sulphuric acid, a blue colour appears.
Cotton-wool is pure cellulose. It is a mass of the long hairs that grow from the testas of the Cotton seeds. It is easier to see the cellulose reaction in this than in either Date or Ash endosperm. If, after treatment, the hairs are mounted and examined under the microscope, it is seen that the walls swell in addition to turning blue.
In the endosperm of the Castor Oil seed reserve food is stored as oil. This is readily proved by the osmic acid reaction . The cotyledons are unusually filmy and are in close connection with the endosperm. The caruncle, or partial aril, possibly aids in absorption of water .
All the seeds hitherto described have an embryo with two cotyledons. There is, however, a very large class of plants whose embryos have one cotyledon only.
The large size of Indian Corn (Zea Mais) as compared with the seeds of most cereals, makes it the best to choose for examination.
The grain is the whole fruit, a caryopsis, in which the fruit coat and seed coat grow together so closely as to be inseparable.
On one side of the flattish grain there is a pale area which indicates the position of the embryo . The coat of a soaked grain is easily removed, and the embryo can then be taken out whole. Its removal leaves a hollow in the main mass, which is endosperm. When this tissue is tested the great bulk of it is seen to contain starch, but the nitric-acid test (p . 387) demonstrates the presence of protein in a layer just within the coat. It is this layer, too, that is particularly rich in vitamins . This is so in all cereals and explains why highly refined flour makes less nourishing bread than does the coarser brown.
One part of the embryo is somewhat shield-shaped and is, for this reason, called the scutellum. This is the one cotyledon . The remaining smaller part is rather elongated and cylindrical. The upper half of it is the plumule, and the lower the radicle.
To understand the relation of embryo to endosperm the whole grain must be cut in halves in a plane passing through the axis of the embryo. For this it is well to choose a grain that is symmetrical. This cut gives a median section of the embyro – plumule, radicle, and cotyledon are all exactly halved. In addition it is a great help to have a stained, mounted section which can be examined with a hand lens.
In this view the embryo lies to one side of the endosperm. The outermost layer of the scutellum is made up of cells whose special work it is to secrete enzymes as soon as germination begins. In the mounted section this layer appears as a row of cells, in close contact with the endosperm. It liberates certain enzymes that break down the cell-walls of the endosperm tissue – others follow and act upon the stored starch, turning it into sugar. The sugar passes along the cotyledon to the radicle and plumule. The former points downwards in the grain. The leaves of the latter are enclosed in a sheath which splits as the plumule elongates. When the radicle emerges it also ruptures a sheath of special tissue, which remains attached to the upper part of the root as a collar, or coleorhiza .
The primary root has but a brief existence. Before it dies off, adventitiousgrow from the base of the stem . These form the fibrous root characteristic of all grasses .
In the Date seed the one cotyledon is circular and sheathing.
It absorbs the endosperm store, which is, in this case, cellulose.
As it absorbs it elongates, pushing the radicle and plumule down into the ground.
Its upper part splits, and here the spike of the plumule emerges. In this case the cotyledon is much more active than that of the Maize. It not only grows downwards, but it expands greatly at its tip, which is still within the seed absorbing the reserve food .
The one cotyledon of the Onion is rather like that of the Date, but it is epigeal, grow- ing upwards as well as down- wards. As its tip is still inside the seed it comes up through the soil sharply bent. Whai the bend straightens, the remainder of the seed is usually dragged upwards, with the tip of the cotyledon still within it, still absorbing the reserve food of the endosperm .
These few examples of seed-structure serve to show the very special work of the cotyledons – it is their business to provide food for the growing root and shoot, either directly or indirectly. Directly either by containing some food store in their own tissues or by actively manufacturing food – indirectly by making available some additional supply.
In the bean the hypogeal cotyledons store the food that, as a result of enzyme action, is absorbed by radicle and plumule.
In the Sunflower this is the case also, but the epigeal cotyle- dons turn green in the light and carry on the work of photo- synthesis.
In Maize, Onion, arid Date the cotyledon acts indirectly in this matter of the food supply, absorbing it from the endosperm and passing it on.
In the Castor Oil the work of the cotyledons is indirect in the early stages of germination, when they absorb the food from the endosperm against which they lie so closely. Later they appear above the ground as the first green leaves of the plant and, by carrying on the work of photosynthesis, contribute directly to the seedling’s food supply.