Читать книгу An Introduction to Nature-study - E. Stenhouse - Страница 14

1. Opposite leaves.—Examine a deadnettle plant (Fig. 92). Do the leaves come off the stem haphazard? How many come off at each level? Are both leaves on the same side of the stem, or opposite each other? Are the two leaves at the next level above just over these, or do the directions cross? Do the leaves get as much light or more than they would if each pair were just over the pair next below? How many other plants do you know which have leaves arranged in this manner? Examine leafy twigs of sycamore, horse chestnut (Fig. 40), and ash. Are the stalks of the lower leaves of these twigs of the same length as those of the upper ones? Is it any advantage for the lower leaves to have longer leaf stalks?

2. Box leaves.—What is the arrangement in the box? Examine particularly the young leaves near the end of the twig. Are the lower ones twisted? Can you suggest a reason for this twisting? Can you find any twigs in which no twisting has taken place? Are these untwisted twigs so placed that they are equally exposed to the light on all sides?

3. Alternate leaves.—Examine the arrangement of the leaves on a wallflower stem. They come off alternately, each springing from a rib on the stem. How many ribs are there? Look at the bottom of the stem, where the leaves have fallen off, and notice that each has left a scar. Mark one of the scars with your pencil and then count how many scars you pass before coming to another on the same rib. How many times do you wind round the stem in doing this? You pass five leaves and wind spirally round the stem twice. This is always the case in the wallflower.

Examine leafy twigs of oak and pear trees. Here, too, the leaves are alternate, and every sixth leaf is above the first, and a line joining all the leaf-bases or scars between the first and sixth leaves would wind spirally round the stem twice.

What is the arrangement in the elm and lime?

4. Leaves which form a rosette.—Examine plants of primrose (Fig. 81) and daisy. The leaves in these cases spring from close to the ground and form a rosette. Notice that the bottom of the leaf blade is much narrower than the upper part. Is any saving of material obtained by this arrangement?

5. The position of branches and buds.—Look in the upper angle between a leaf and the stem in all your specimens. This angle is called the axil of the leaf. Can you see a bud in the axil of the leaf? Can you find that a bud or a side branch ever arises in any other position? (The former positions of fallen leaves are marked by scars.)

To the beginner in nature-study leaves seem in the majority of cases to be arranged on the stem of a plant in a haphazard and confusing manner, and it is only after very careful observation that a definite order and regularity is seen to be always maintained.

Nodes and internodes.—The level at which a leaf springs from the stem is called a node (Lat. nodus, a knot), and the length between two consecutive nodes is called an internode (Lat. inter, between).

Opposite leaves.—It is best to begin the study of leaf-arrangement by examining some such plant as the deadnettle (Fig. 92). The leaves come off in pairs: two at the same level, set opposite to each other. The next pair above or below springs from the stem in a direction at right angles to the first—a device which allows the leaves to get a more equal share of light than if each pair were placed directly over the next below.

A similar arrangement is adopted by various other plants, including the horse chestnut (Fig. 40), sycamore (Fig. 34), box, privet, etc., but in some instances it is disguised. Box twigs afford an interesting example of this. Those twigs which are equally, or almost equally, illuminated on all sides, have their leaves arranged in pairs at right angles to each other like those of the deadnettle. Some twigs, however, receive the light from one direction only, and in these cases the leaves turn themselves until they face the light; so that at a casual glance the pairs of leaves seem to lie all in the same plane. One only needs to examine the end of the twig, where the leaves are just unfolding, to see that the arrangement is really in pairs alternately at right angles. In the case of the privet the efforts of the leaves to face the light often cause the stem itself to be twisted between the leaf-levels.

The alternate, or spiral, arrangement.—Perhaps the commonest leaf-arrangement is one in which only one leaf is given off at any particular node, the next leaf being a little further round the stem, and so on. As a result, an ink line or a piece of thread joining the leaf-bases would wind spirally round the stem. In the case of the wallflower, oak, pear, and many others, such a line would wind spirally twice round the stem before coming to a leaf vertically above the first, and in so doing it would pass five leaves. This may be shortly described as the ²/₅ arrangement. A less common one is ⅜, where in winding spirally round the stem 3 times, 8 leaves would be passed.

Efforts of leaves to obtain light.—It would be difficult to imagine any order of insertion which would secure a more equal distribution of light to each leaf than the spiral arrangement; but here also cases of leaves twisting in order to face the light are not uncommon. Lime leaves very often turn for this reason, so as to lie in almost the same plane—adopting a device similar to that of the box and privet described above. Further, the lime leaves arrange themselves at such angles that there is very little overlapping. Elm twigs also often exhibit similar instances of a mutual accommodation of leaves to each other’s light supply.

The lower leaves on a horse chestnut twig have longer stalks than those nearer the end (Fig. 40). This enables the leaves to stand well out to the light and escape the overshadowing of those above.

The positions of branches.—A branch of the stem of a flowering plant always arises as a bud in the upper angle between a leaf and the stem. This position is called by botanists the axil of the leaf, from the Latin word axilla, the arm-pit. Clearly, then, the arrangement of the branches is primarily dependent upon that of the leaves, and we shall, for example, never find “opposite” branches on a tree which bears its leaves on the “alternate” system. It is easy to notice, however, that not all the buds develop into branches. In other words there are many buds which remain dormant, and the final arrangement of the branches is often somewhat irregular on this account. But wherever an ordinary bud or a branch occurs, we may be perfectly sure that there was once a leaf immediately below, even if the leaf-scar can no longer be seen.

Economy of leaf surface.—All these things seem to indicate that a good supply of light is of the greatest importance to leaves, and this conclusion is supported by the fact that leaves are usually either narrow or actually cut away in places where the light cannot reach them. The leaves of the daisy and of the primrose (Fig. 81), for example, all spring from nearly the same point, and form a rosette. Evidently there would be a certain amount of overlapping at the leaf-bases, unless the blades there were very narrow, as they are. Again, the greatly-indented leaves of the ivy are often arranged so that a point of one leaf fits over an indentation of another—a beautiful example of plant economy.