Chapter 49
Chapter 47 of House, Garden, and Field by L.C. Meehl. This LibriVox recording is in the public domain. Grasses. The Characters of Grasses. By what marks do we recognize grasses? I suppose that most of us would say that any plant is a grass which has long, narrow, pointed leaves, hollow stalks, halms, and small, greenish flowers. So different are grasses from all other plants that we should have no hesitation in deciding whether a single leaf, a single home, or a little
cluster of flowers belong to a grass or not. When we look closely, it is easy to find further differences between grasses and other plants. The base of a grass leaf forms a sheath around the home, which runs down the stem to the knot next below, and is nearly always split. Just at the place where the blade becomes free, there is a little colorless scale, which is in close contact with the hum. The leaf is generally ridged on its upper surface, and if we cut it across and look at
the cut edge with a lens, angular ridges will be seen. The hollow hum with knots at intervals is almost equally distinctive. The flowers are usually very numerous and very small, so that it is not easy to make out all the details, but in a flowering grass we can see two things which are peculiar, lightly poised anthers, which hang out and dance in the wind, and long, slender, feathery styles. The only plants which come so near to grasses that a doubt can arise as to whether they are
grasses or not are certain sedges and rushes. In these, the sheathing leaf bases are either wanting or not split, and there is no colorless scale. The stalks are commonly filled with pith, the anthers of the stamens are erect, and do not dangle as in grasses. The numbers of the flower parts are also in many cases different from what we find in grasses, where there are nearly always three stamens and two styles. There are many sorts of grasses, and about a hundred
species grow wild in the British Isles. A very little attention will show that in every hayfield there are several distinct species with quite different flowers. Any grass that we happen to examine will suggest a number of questions, and it may easily happen that among these will be some that we cannot answer to our satisfaction. It is a good practice, however, to put questions incessantly,
for it is chiefly in this way that we make progress in the interpretation of natural objects. Why are grass holmes hollow and jointed, that is, with solid partitions at intervals? A hollow cylinder, like a grass holm,
is better able to resist bending than a solid stem of the same weight for a given length. Take two lumps of plasticine or modeling clay of the same weight. Shape one into a solid cylinder. Spread out the other into a flat sheet and roll it up till its edges meet.
You can thus get two cylinders of the same length and the same weight, one solid and the other hollow. Lay each upon two supports, the distance between the supports being the same in each case. Then test the power of the two cylinders to resist bending. A tape holding up a suitable weight may be hung from the middle point of each cylinder. the result will leave no doubt as to the greater resistance to bending of the hollow cylinder.
For the reason of the different resistance to bending of the two cylinders, I may refer the reader to round-the-year article, Haytime. The hollow grass halm is light, strong, and springy, yielding easily to wind without being damaged by it, except indeed when the seeds are nearly ripe and the top of the halm is heavily loaded. Then wind and rain may lay the halms flat, but even for such an accident, a remedy is provided as we shall shortly see.
Solid partitions or knots mark the places where the bases of the leaf sheaths are attached to the hom. Here, the vessels pass out into the leaves, and it is chiefly the interwoven vessels which form the knot. The solid partitions stiffen the hom and hinder it from becoming flattened by pressure. But there is another and less obvious reason for the knots. Take an entire grass plant fresh from the ground and a foot or more in height. Plant it in a tray of wet earth or sand,
not upright but horizontal, and see what will happen. A very top-heavy grass will not do. If the experiment is made with care and judgment, you will see in the course of a day or two that the hum begins slowly to erect itself. Each segment between two neighboring knots sets itself at a small angle to the segment next below, and as all the angles are bent towards the same side, the horizontal stem soon begins to rise.
Before long it would be found to have completely erected itself and perhaps to lean over a little to the opposite side. You can hardly fail to remark that all the bending necessary to erection is affected at the knots and that the intervening parts of the stem are nearly straight all the time.
There is evidently at each knot what we may call an organ of movement, compared to clover and wood sorrel. If you mark one of the knots with horizontal India ink lines passing around it a small distance apart, say one millimeter, you will see that in a day or two the lines become a good deal wider apart on the side from which the home is bending. The organ of movement changes its form, swelling on
one side and either not swelling at all or swelling to a less extent on the other side. This power of swelling unequally, according to circumstances, is due to absorption of water. The knot, or some structure in communication with it, evidently possesses sensibility. It can feel, so to speak, when it is displaced, and absorb so much water as to bring the hom back to the upright position.
Why do the bases of grass leaves and sheath the hom? While the grass hom is still growing, the outer leaf sheaths protect the inner ones, and the inner ones protect the hom. As the hom attains its full height, the inner parts are gradually withdrawn from the outer ones like the joints of a telescope,
and the sheaths become free from one another. A young and soft shoot is stiffened, being made up of a nearly solid mass of sheaths, one within another, but an older and firmer shoot is hollow, light and springy, and needs no support from the leaf sheaths. No better plan could be devised for the rapid lengthening of the flowering stalks. Something too is gained by carrying higher the base of the free leaf blade, for to overtop its rivals is a leading feature in the policy of most
grasses. Why are the sheaths of grass leaves split along one side? To permit of expansion without tearing as the parts within enlarge. The hom within the sheath rapidly expands in diameter as it becomes older. Sometimes a growing ear or mass of flowers is lodged within a leaf sheath and needs room for its expansion. Why are most grass leaves ridged on the upper surface? The ridges when cut across are seen to be more or less triangular and fit neatly together when the leaf is rolled up.
Make a model of a grass leaf by gluing triangular bars of wood to a strip of canvas and see how neatly such a model can be rolled up or expanded as circumstances require. Nearly all grass leaves are rolled up in their early stages of growth, and even when full grown, they may require to be rolled up as a temporary protection against hot sun and dry air. Some of our native grasses growing on dry pastures, such as Cisleria, can roll or unroll in a few minutes. It is enough to put a
bell glass over the growing plant to cause the leaf to open widely, as it always does when the air contains much moisture. If we remove the bell glass and expose the plant thereby to the warm, dry air of an ordinary room, the leaf will roll up again and expose a diminished evaporating surface. Some grasses, like the matte grass of our moors, nardis, or the sheep's fescue, grasses which inhabit places where there is no shelter from the sun and wind, are permanently inrolled. Others,
which grow in damp meadows or shady woods, never roll up when they have once expanded. A few grass leaves are flat and have no ridges at all. The stomates of a grass, that is the pores by which water vapor is given off and air taken in, often lie only on the upper surface of the leaf, within the grooves between the ridges. Hence, they are well protected from too dry air, especially when the leaf is wholly or partially rolled up. If the leaves are flat, the stomates
are usually found on both surfaces. In certain cases, this concealed position of the stomates protects them against an opposite but equally dangerous accident, that of being choked by water, which would prevent gas or vapor from passing in or out. You have no doubt often seen the float grass, glyceria flutens, rooted in the mud and spreading out its leaves, which are sometimes yards long,
upon the surface of a pond or a slow stream. The leaves of float grass lying flat on the water could not, if they possessed the ordinary leaf structure, drain off the rain, and if they happened to get splashed or drawn beneath the surface by a current, we might suppose that they would find it very hard to get dry again.
But no such difficulty is met with. The leaf of the float grass, no matter what is the state of the weather, no matter how roughly the leaves have been treated, is always dry on its upper surface and always wet on its undersurface. The dryness of the upper surface is due to the deep furrows between the ridges,
and to these the surface film of the water cannot pass. See Object Lessons from Nature, Part 2, pages 135 and 6. and the water above the surface film is accordingly held up and prevented from entering. No accident to which the float grass is exposed can fill the furrows with water or drench the stomates which lie sunk in them.
There is another glyceria, almost equally common in watery places. In this second species, glyceria aquatica, the leaves never float, and it is interesting to remark that they have no ridges on their upper surface. It is probable that grass leaves originally became ridged on their upper surfaces to facilitate rolling up lengthwise during seasons of drought. But float grass has turned its leaf ridges to account as a means of preventing the wetting of the stomate-bearing surface.
A cross-section of the leaf reveals a number of enclosed air spaces, which, one would think, must greatly increase the buoyancy of the floating leaves. However, in the second species of glyceria, G. aquatica, whose leaves do not float, the airspaces are much larger. They are not simple cavities, but are filled with stellate cells. This was pointed out to me by Mr. Norman Walker in sections of G. flutens.
Mr. Luton-Brain, Transactions of the Lenean Society of London, 1904, says that the low ribs of G. flutens probably have no significance as an adaptive character. I suspect that he has not seen sections through floating leaves, where the ridges are as sharp and distinct as possible.
In aerial leaves of the same species, the ridges are much lower, especially in the neighborhood of the midrib. What is the use of the colorless scale which is found inside the leaf sheath, just where the blade becomes free? I have puzzled over this question for years without the least success.
Some people think that the scale hinders water from making its way into the sheath. it is an objection to any such explanation that the surface film of water cannot pass into narrow spaces bounded by unwettable surfaces, so that a scale does not seem to be necessary to hinder it from passing in here. I took three common grasses, cut off the leaf blades and their scales, ligules, and immersed them in water. No water made its way into the leaf sheaths. The scale is very
constant in true grasses and peculiar to them. Why do the anthers of grass flowers dangle? In order that the wind may shake the pollen out of them more easily. In hay time, the air carries everywhere the minute pollen grains of grasses, and at this season, the dust which settles in still places always contains grass pollen. Grasses are wind pollinated. Why are the styles of grass flowers long and feathered? In order that they may have a better chance of catching some of the
pollen grains which are wafted past by the wind. Why are grass flowers small, inconspicuous, and greenish? Because the grass has no need of insects or other animals to pollinate its stigmas. Remark the differences between a flower which is wind-pollinated and one which is insect-pollinated. We may take any common grass as an example of the first kind. Red clover, primrose, convolvulus, rhododendron, and orchids are familiar examples of the other kind. Wind-pollinated
flowers, 1. are inconspicuous, 2. are scentless, 3. secrete no honey, 4. produce much pollen, most of it being wasted, and 5. often have feathered stigmas. Insect pollinated flowers are 1. usually conspicuous, 2. are often scented, 3. usually secrete honey, 4. produce less pollen, comparatively little being wasted, and five usually have simple stigmas. In certain states of the weather, grass leaves exude much water. There are fissures in them by which drops of water can be
passed out. It seems that a low temperature is particularly dangerous to green tissues which are laden with water. During a warm day when rain has saturated the earth, absorption of water goes on freely. Even after sundown, the ground may still be warm enough to favor rapid absorption by the roots, but the air cools fast, and a temperature low enough to be dangerous to the softer tissues may obtain only a few inches above the warm soil. Under such circumstances, grasses and other herbs
pass out the water, which has become superfluous and even dangerous, in the form of big drops. Then people generally say that there has been a heavy dew, though it may be that the sky was overcast and that no dew would ever fell. Exuded water may be distinguished from real dew by attending to two points of difference. Dew never forms except under a clear sky. Exudation takes place whenever plants gorged with water are exposed to cold air, whether the sky is clear or cloudy.
Secondly, dew forms as minute, close-set drops, which on a surface not easily wetted may afterwards run together to form big drops. The drops exuded from the water pores of leaves, on the other hand, are big and solitary from the first. The exudation of drops from grass leaves can be brought about at pleasure. Cut a sod, damp it, lay it on a glass plate, and cover it with a bell jar. In a day or so, the grass, kept at the temperature of an ordinary room, will exude abundantly from the leaf tips.
End of chapter 47
