1 6 PLANTS 



cellars, caves, hollow logs and similar dark situations where 

 green plants are never found. 



2 2. There seems, therefore, to be a direct relation between 

 the green color of plants and the sunlight, and this becomes 

 still more evident when we consider the distribution of the 

 green on the individual plant. In the smaller herbaceous 

 plants the green may be found in all parts above ground, in 

 stem and leaves alike; but in the larger perennial growths, like 

 the oak tree, the green is found only in the leaves and twigs; 

 possibly also on the surface of the smaller branches and between 

 the ridges of dead bark on the larger limbs and the trunk itself. 

 But in the dark central parts of the branches and trunk, and 

 beneath the thick ridges of dead bark there is no green. It 

 may be wanting entirely in the stem, but, with the rarest 

 exceptions, it is always present in the leaves. 



The Leaves 



23. In connection with its color it is also important to note 

 the form of the leaf. This varies through an infinite variety 

 of patterns from circular to hnear, but in almost every case it 

 is either very small, or else very thin or very slender in propor- 

 tion to its other dimensions. From this it results that the 

 surface of the leaf is large in proportion to its volume, and all 

 of its substance lies near the surface, that is, exposed to the 

 light. In other words, the form of the leaf is such as to give a 

 maximum exposure of its substance to the light. As a general 

 characteristic of leaves this one of form is second in importance 

 only to that of the green color. 



24. A typical leaf consists of three parts: (i) a broad, thin 

 portion — the blade, (2) a narrow rounded or angular stem — 

 the petiole, and (3) at the base of the petiole a pair of wing-like 

 appendages— the stipules. Stipules vary greatly. They are 

 usually more or less leaf-Hke, but may be reduced to mere 



