396 DISTRIBUTION OF THE GREEN LEAVES ON THE STEM. 



2. THE GREEN LEAVES. 



Distribution of the green leaves on the stem. — Relation between position and form of green leaves. 

 — Arrangements for retaining the position taken up. — Protective arrangements of green leaves 

 against the attacks of auiinals. 



DISTEIBUTION OF THE GREEN LEAVES ON THE STEM. 



Landscape painters tell us how difficult it is to treat foliage correctly, and at the 

 same time artistically; how hard, for instance, so to reproduce the leafy crown of 

 maples, beeches, elms, limes, and oaks that they shall immediately be recognized 

 for that which they are intended to represent, and at the same time that that effect 

 and tone shall be produced which is aimed at in the picture. The variety of the 

 foliage is caused not least by the distribution of the green leaves on the branches, 

 and by the branching dependent upon this; things as definite as possible for each 

 species of tree, and, generally speaking, for every plant. 



On cutting various leafy branches and observing the distribution of the leaves 

 on them, the following difierences first strike the eye. In numerous plants it is 

 seen that two or more leaves originate at the same height on a branch, while in 

 many other plants, at a particular level of the stem or branch, only a single leaf 

 is produced. In order to be able to understand these circumstances, it is advisable 

 to imagine the leaf-bearing shoot or stem as a cone. The apex of the cone 

 corresponds to the upper end, and the base of the cone to the lower portion, i.e. to 

 the oldest part of the shoot. The whole shoot is not at any time in a com.pleted 

 state; it continues to grow at the apex, and at the upper part is not only younger, 

 but is also less bulky than the older portions lying nearer to the base. It can, 

 therefore, indeed be quite well compared to a cone, although this form is only 

 seldom so noticeably to be met with as in the following diagrammatic figures. 



That which applies to the age of the various portions of the shoot naturally 

 applies also to the leaves projecting from the shoot. That is to say, the lower 

 leaves of the shoot are the older, the upper leaves are the younger. On looking at 

 the top of the cone (see fig. 98), the places of insertion of the older leaves appear to 

 arise, first of all, from the circular disc which forms the base of the cone, while the 

 younger leaves originate close to the apex, therefore close to the centre. The stem 

 is to a certain extent divided up by the leaves into sections one above another. 

 Usually it is somewhat thickened or knotted at those places where the leaves 

 project from it, and therefore the places of origin of the leaves are designated as 

 nodes. Each portion of the stem lying between two successive nodes is called an 

 internode. When two leaves project at the same height from the stem, they are 

 inserted opposite one another, not unlike the two extended arms of a human body, 

 and they appear on the cone-shaped stem (whose cross section at all heights 

 forms a circle) at a distance from one another of exactly half the circumference 

 of the circle (180°), (fig. 98^). If three leaves spring together from the stem. 



