ABSORPTION of water is the primary function of. The roots of land-plants, as they grow through the soil, turn their tips definitely from a dry spot in the ground to one that contains more moisture.
It is for this reason that a mere sprinkling of gardens in dry weather is a mistake. The roots tend to abandon their normal direction of growth and turn upwards to the wet region. Watering, therefore, except in the case of, should either be done with extreme thoroughness or left alone.
This particular irritability of roots (and by “ irritability” is meant their tropism, or response to some outside stimulus) can be demonstrated by germinatingin gravy strainers whose base is a fairly coarse sieve. On a thin layer of sawdust or coco-nut fibre, that must be kept uniformly damp, mustard or cress are sprinkled. Each sieve is then placed over a beaker containing water. In three or four days the seeds are germinating. A day or two later the roots are pushing through the holes in the sieve and growing vertically downwards.
If one sieve is now removed and put upon an empty beaker, the roots turn upwards and actually re-enter the holes of the sieve, responding to the attraction of the wet sawdust or fibre. The roots of the other set ofcontinue their downward growth towards the water in the beaker.
This is a striking experiment to show the attraction that water has for roots and their response to this particular stimulus.
In a fully grown plant there is a certain co-ordination between the arrangement ofand the spread of roots, so that the former help to conduct water to the latter. For this reason a spreading tree makes a good shelter in a sharp shower. The rain that falls on the upper part of the tree is, for the most part, conducted from the tips of the of the upper branches to the branches below, which extend outwards a little farther. Likewise from the tips of these branches it drips to those immediately below, which extend outwards a little farther still. So the drip falls to the ground, following the course of a cone or pyramid, and leaves the area immediately under the crown of the tree comparatively dry.
But the branches of the roots underground stretch as far as the-bearing branches overhead. Further, it is near the tips of roots that water is absorbed. Thus the steady drip from the leafy branches conducts the rain to the area where it has only to soak downwards to reach those parts of the that absorb.
This co-ordination between leaves andis not a special feature of woody perennials, but is common, too, among herbaceous plants.
In the Arum Lily the tips of the spreading lateral roots occupy ain the soil just underneath the tips of the smooth, shining leaves. The leaf-tips bend outwards and downwards and thus conduct rain-water directly to the absorbing tips of the roots.
In Rhubarb the root grows straight down into the soil, with but few branches, and these are not spreading. The form of the leaves is such that they conduct water inwards, not outwards as in the Beech and Arum. The Rhubarb leaves are grooved on the upper surface. Water travelling down this groove continues its course along a groove of the leaf-stalk that is familiar to everyone who has peeled Rhubarb. By this route the rain is definitely conducted to the centre of the plant and soaks into the ground, just above the downwardly directed root.
The same co-ordination is seen in all bulbous plants.
The upstanding leaves of Tulip,, and conduct the rain to the centre, where the bulb rests just below the soil and gives off its adventitious roots.
When radical leaves grow in the form of a rosette, as in the Dandelion and many other plants, rain is also centripetally conducted by the grooved, upper surfaces of the leaves. It soaks into the soil just where the long, straight root grows downwards.
The gardener’s root-of Apple and other trees helps to maintain the right relation between the pruned branches overhead and the spread of the roots in the soil.
It is, generally speaking, only the region near the tip that is the absorbing area of a root. The absorption depends very largely upon numbers of fine root-hairs that grow in this region. It is useless to look for them on an uprooted plant, for various reasons : they have been destroyed by the pull ; they are hidden by the clinging particles of soil ; they have shrivelled in the dry air.
They can, however, be seen on the seedlings in the gravy strainers or, better still, on young bean roots grown in a damp atmosphere. Clean gas-jars, each containing a single roll of clean blotting-paper, make excellent apparatus for the demonstration of root-hairs. Not more than two soaked Broad Beans should be slipped between the dry blotting-paper and the glass in each jar, one on either side. It is neater so to arrange thethat the black mark, or hilum, of the coat is downwards. Water, not more than an inch or two in depth, should then be put into the jar. When the germinates, its root is in a damp atmosphere, and this is exactly the right environment to induce the growth of root-hairs.
It is to promote the rapid growth of root-hairs that plants must be watered freely when they have been transplanted. The uprooted plant has lost its root-hairs. In the damp atmosphere of a well-watered patch of soil new root-hairs are produced. These at once begin their work of absorption and the droopingand leaves revive.
The plants produce no hairs on any part of the root that is actually in water. It is only in two or three exceptional cases that they are present on the roots of water-plants. Water-plants take in water through the outermost tissue of the absorbing region of young roots.
The number of root-hairs varies greatly on different roots and under different conditions of. The variation ranges from the zero of water plants to as many as 400 to the square millimetre on roots of Indian Corn grown in a moist chamber. This number has actually been counted.
One great advantage of the presence of root-hairs is, undoubtedly, the great increase that they give in the absorbing area of the root. It is estimated that in some cases the area of absorption is increased as much as eighteen times, by the presence of root-hairs.
A greatly increased area of absorption is gained by some roots because of their close association with certain fungi. In such cases the roots are closely invested by some of the exceedingly fine fungal threads, while other threads spread outwards in the humus of the soil, and absorb food compounds. Such a fungal investment is a mycorrhiza. Beech andhave adopted this method of absorption from the soil.
It is difficult to demonstrate absorption in the root-hairs themselves. It is necessary, therefore, to have recourse to analogy to make the process clear.
In the physical process of osmosis two liquids, of different density, separated by a permeable membrane, exercise a mutual attraction for one another so that each passes, in the reverse direction, through the separating membrane.
In the laboratory, to make the experiment as striking as possible, it is well to use substances of widely different strength : golden syrup, for instance, and water.
A piece of parchment, or pig’s bladder, should be tied very tightly over the mouth of the bulb of a thistle funnel, and the bulb then exactly filled with syrup. This process needs care. In pouring the syrup the tube should be held in a sloping position, so that the air in the funnel may escape along the upper side of the tube as the syrup glides down the lower side.
The thistle funnel is then clamped with the bulb suspended in a beaker of water. In a very short time a rise is noticed in the level of the liquid in the bulb. The rise continues up the tube to such an extent that, if it is wished to continue the experiment, a second glass tube must be attached, by means of rubber tubing, to the end of the thistle funnel.
Obviously water has passed inwards through the permeable membrane. At the same time, however, the water in the beaker has become a little sweet. It is only necessary to taste it to prove this. There has, therefore, been an outward flow of syrup through the membrane. That is, exosmosis, as well as endosmosis, has taken place.
This experiment can be varied by using other solutions : a solution of salt, for instance, or of copper sulphate. Also the stronger solution may be in the beaker and the weaker in the funnel. No matter what the arrangement, there is an interchange of liquids through the membrane.
Osmosis in Living Cells
This process has taken place in apparatus made up of non-living parts. It is easy to prove that it can take place, to some extent at any rate, in living cells. With the exception of the outside skin, a potato is completely made up of living cells. For this reason, and because of its bulk, it makes a good piece of apparatus for demonstrating osmosis in living tissue.
In order to have a control experiment two large potatoes are used. Each has a piece cut off one end, so that it may stand on a flat base. Above the base a ring of skin is peeled away. A pit is made in the middle of each potato, stopping short within an inch of the base. They are now put into separate dishes, so that each stands in an inch or so of water. One pit has a little sugar put into it ; the other is left empty.
The sugar dissolves in escaped moisture and forms a solution at the bottom of the pit. Very soon a rise in level is noticed here, as in the glass tube of the thistle funnel. Attracted by the sugar solution, water has passed from the cells that form the sides of the pit. Owing to loss of water the cell-sap in these particular cells becomes stronger than it is in neighbouring cells beyond these. This stronger cell-sap now attracts water from neighbouring cells beyond them, so that they, in their turn, contain a stronger sap than the cells immediately beyond them. So a chain is started which ultimately affects the cells quite on the outside of the potato, and they attract the water of the vessel in which it stands. It was to allow this water to enter that a little of the potato skin was peeled away, because the cork layer of the skin would otherwise hinder the entrance of water.
There is, then, a gradual rise of liquid in the pit of the potato, because osmosis has taken place through its living cells. In the control experiment there has been no such rise ; the pit is completely dry.
Osmosis Under the Control of Protoplasm
Another very striking experiment may be carried out with a cube cut from the middle of a fresh beetroot. The red sap that oozes from the cut cells should be removed r by washing in tap water. When the cube is now dropped into a beaker of cold water there is no outward flow of red sap.
If the same cube is killed by being dropped into boiling water, then transferred once more to the beaker of cold water, the red sap flows out freely. This proves, beyond all doubt, that there is something present in the living cells that keeps them from losing their solutions.
This “ something “ is the primordial utricle, a layer of protoplasm that lines the cellulose wall of each young cell and, by its action, modifies osmosis.
It is upon osmosis, modified by living protoplasm, that the absorbing capacity of root-hairs depends.
A root-hair is one cell of the outermost tissue of a young root. The tissue is called the pihferous layer, because it produces the “ pile,” or hairs. Each hair is formed by the outside wall of a cell growing out for some distance, at one point only. The cell-sap within the hair is a slightly acid mineral solution, but, as it is stronger than the mineral solutions in the surrounding soil, these are attracted through the cell-wall and enter the cell.
Roughly, then, the root-hair cell corresponds to the thistle funnel with syrup, and the soil and its mineral solutions are represented by the beaker and water.
The difference in the osmotic process in the two cases depends upon the living protoplasm. The cellulose wall and the primordial utricle lining it, together form a semi-permeable membrane. The result is that, while there is a very definite endosmotic current, only a very small proportion of the contents of the cell is allowed to escape outwards, into the soil.
Through neglect a potted plant may droop so much that it appears almost lifeless. Unless the neglect has been too gross the plant invariably recovers on, especially if tepid water is used, because roots absorb warm water more readily than cold. Endosmosis takes place more quickly under these conditions, the individual cells become entirely filled with water, and the drooping leaves of the plant are restored to their normal turgidity.
In England, in winter, a window plant sometimes suffers because it is “caught by the frost.” The droop of its leaves is much like the similar state in a neglected plant. In this case, however, norestores the freshness and turgidity of the leaves. The frost has killed the primordial utricle of the living cells. Under such conditions the cells can never become turgid.
Were it not, then, for the intervention of the primordial utricle, there would be no such thing as turgidity in plants. But the growth of cells depends upon the preliminary stretching of their walls due to their turgidity; therefore if there were no turgidity there would be no growth. We come, then, to this logical conclusion : In the absence of control by the protoplasm of young cells, life, as we know it, at any rate, would cease to exist.