4i4 



SCIENTIFIC NEWS. 



[Oct. 19, i? 



WORK FOR NATURALISTS' CLUBS. 



IV. — Leaves. 



COLLECT a number of leaves of various kinds, 

 deciduous and evergreen, long and round, simple 

 and cut. Elder, laurel, hyacinth, lilac, tropseolum, ash, 

 horse-chestnut, and grass will make a good selection. 



Notice the effect of situation upon the form of the 

 leaves. Social plants, such as grasses and daffodils, 

 have often narrow leaves, which do not shade one 

 another too much. Here and there a masterful intru- 

 der, like the daisy, dandelion, or shepherd's purse forces 

 its way in among them, forming the selfish arrangement 

 known as a rosette. A circle of broad leaves pressed 

 tightly against the ground keeps a clear space which com- 

 petitors cannot enter, If you cut out the root you will 

 see how effectually the rosette-forming plant keeps its 

 little patch of ground to itself. Leaves carried upon the 

 boughs of trees are often much divided, so as to bend 

 and fold together in a high wind. Floating leaves and 

 leaves of marsh-plants are usually of very simple out- 

 line, for they have few competitors, and are little liable 

 to get in one another's light. Submerged leaves are 

 generally cut into narrow segments, but the reason is 

 not very clear. Some naturalists suppose that they need 

 to expose as large a surface as possible, in order to 

 extract the minute proportion of carbonic acid dissolved 

 in a vast bulk of water. Lubbock (" Flowers, Fruits, and 

 Leaves") and Grant Allen have discussed the shapes of 

 leaves in a very interesting and profitable way. 



The surfaces of leaves give a wide scope for observa- 

 tion and discussion. How much there is to tell about 

 hairs ! There are glandular hairs to catch or deter small 

 creeping insects ; hairs set in weels, as effectual a pro- 

 tection against young animals as a spiked palisade 

 against roving boys ; hairs which keep off browsing 

 quadrupeds, like the hairs on some common grasses, 

 which cattle never touch ; then there are hairs which 

 keep the little drops of moisture (dew-drops, for ex- 

 ample) from dispersing, and force them to run together 

 into big drops, which roll down the channelled leaf- 

 stalk, and so wet the earth about the roots ; lastly, there 

 are hairs which, like those of the nettle, are poisoned 

 stings. We forget the vexation of the nettle-rash when 

 we observe the ingenuity of the mechanism by which it 

 is caused. The nettle-hair is provided with a brittle 

 and pointed siliceous cap, which breaks off in the wound. 

 Then the poison is able to flow out through the tubular 

 hair from a reservoir at its base, and aggravate the tiny 

 puncture. We cease to wonder that nettles are let alone 

 by nearly all animals except jackasses, when we con- 

 sider how admirably it is protected. The success of the 

 invention is attested by the number of fraudulent imi- 

 tations. Ever so many " dead nettles," plants with the 

 outward form of the nettle, but innocent of offence, 

 profit by their artful resemblance to the dreaded stinging- 

 nettle. I 



But we must get away from the hairs of leaves, 

 if our article is to keep within any reasonable limits. 

 Kerner, on " Flowers and their Unbidden Guests," is 

 well worth looking into on the subject of defences 

 against gnawing insects. The leaves of evergreens are 

 protected by their glossy surface from rain and dew, 

 which might freeze upon them, to the great injury of 

 the tissues within. Half-melted snow cannot be kept 

 off so easily, and this, followed by frost, is sometimes 

 very deadly. 



The stomates of leaves are easily found. Notch the 

 surface of a succulent leaf, say hyacinth, with a razor or 

 sharp knife. Then catch the edge of the surface be- 

 tween the blade and the thumb. Strip off a small piece 

 of the epidermis, and mount in water. If the air- 

 bubbles are troublesome, momentary warming over a 

 spirit-lamp, or the addition of a drop of alcohol, will be 

 useful. Common books describe the structure of the 

 stomates, but they often give a mistaken account of their 

 use. The stomates serve for the regulated escape of 

 water-vapour, not for the absorption of gases. It has 

 been experimentally proved that the upper surface of the 

 leaf absorbs quite as much carbonic acid as the lower, 

 and yet it is the lower which carries nearly all the 

 stomates. The mechanism for regulating the orifice of a 

 stomate depends upon the power which the guard-cells 

 possess of rapidly absorbing water when stimulated by 

 sunlight, and upon the ease with which they give up 

 their water in the dark. A simple model will make the 

 action of turgidity in opening the stomate quite obvious. 

 Fix to an upright board a loop of good indiarubber 

 tubing. One end of the tube is to be clamped to the 

 board, so as to hold and close it at the same time. 

 Secure the top of the loop by a hook and pass the other 

 end of the tube through a brass tube, tight enough to 

 prevent it from slipping, but not so tight as to choke the 

 passage. The two limbs of the tube should be drawn 

 into a parallel upright position, when they will roughly 

 represent the guard-cells of a closed stomate. Now pass 

 the open end over the nozzle of a water tap (if a good 

 pressure is at command) or of an oxygen bottle. Apply 

 the pressure gradually, and observe how the two limbs 

 of the tube separate from each other as they become 

 distended. 



Then sections through leaves and leaf-stalks will, of 

 course, be studied. Bower and Vines' "Practical 

 Botany " may be recommended as a useful guide to the 

 demonstration. The epidermis, with its hairs and 

 stomates, the palisade cells towards the upper surface (in 

 " bifacial " leaves), the spongy tissue towards the lower 

 surface, the air-spaces, and the vascular bundles, will 

 easily be made out in a good, thin section of the leaf. 

 The palisade-cells, set at right angles to the surface, 

 permit the chlorophyll-corpuscles to avoid exposure 

 to an over-intense light by ranging themselves along 

 the side walls of the cells. If we were troubled with 

 a light too intense for our eyes, and had no notion of 

 making window-blinds or curtains, we should probably 

 alter the shape of our rooms, especially if bricks were 

 semi-transparent, like cell-walls. We should build long 

 and narrow rooms, lit only at one end, and take care 

 not to sit facing the light. This is what the chlorophyll 

 corpuscles do, and the palisade-cells give them every 

 facility for executing the manoeuvre. Observe that 

 upon the very same tree, leaves fully exposed to the 

 light become thickened, while those that are half-shaded 

 remain thin as fine paper. The difference is very appa- 

 rent in the beech. By thin sections, it can be shown 

 that the thick leaves have three or more rows of pali- 

 sade cells, the thin leaves often only one. Where a leaf 

 is exposed to full light on both sides, as is the case with 

 the erect leaves of the hyacinth or daffodil, it becomes 

 " centric," i.e., palisade-cells are formed on both sides. 



In the leafstalk, notice the vascular bundles, the 

 tough and elastic collenchyma (cells with thickened 

 angles), and the air-spaces. If a good section is got, 

 it will be vastly improved by treatment with Schultze's 



