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.