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 [Pg 46] 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 [Pg 47] twigs also often exhibit similar instances of a mutual accommodation of leaves to each other’s light supply.