234 



LEAVES 



The variations in form and structure of leaves is so great that 

 they are often used in classifying plants, and for this purpose 

 many technical terms have been devised to describe these varia- 

 tions. Since most of these variations concern only those who are 

 interested especially in the classification of plants, only the most 

 common ones will be considered in this presentation. 



Primary and Secondary Leaves. Leaves may be divided into 

 primary and secondary. The cotyledons are examples of primary 

 leaves. The cotyledons are parts of the 

 embryo and hence precede the stem in 

 development, while the leaves developing 

 later and called secondary leaves arise 

 from the stem. The secondary leaves are 

 usually numerous, while the primary leaves 

 are few in number. Primary leaves are 

 usually short lived and often fall away as 

 soon as their stored food is exhausted. Gen- 

 erally they disappear while the plant is still 

 quite small. Consequently the leaves of 

 plants that attract attention are the second- 

 ary ones, and when the term leaves is used, 

 secondary leaves are usually meant. 



Development. Leaves develop upon the 

 sides of the growing points of stems and 

 FIG. 215. Leaf of first appear as mere swellings, the smallest 

 the Apple. 6, blade; swellings being near the apex. It follows 

 p, petiole; s, stipules; then that the oldest leaves are at the base 



r ' l of the stem or twig. Thus in a Corn stalk, 



for example, the leaves decrease in age from the lowest leaf on 

 the stalk to the highest. Swellings similar to those that become 

 leaves appear later just above the leaf swellings, and these 

 become the buds which appear in the axils of the developed 

 leaves. In woody plants which prepare for a rest period, the 

 leaves are partly developed during the previous season, and rest 

 in the bud in a miniature form until the following spring when 

 they burst from the bud scales and in a few days complete their 

 development. 



Parts of a Leaf. In a typical foliage leaf, such as that of the 

 Apple shown in Figure 215, there are three parts: the expanded 

 portion or blade; the leaf stalk, called petiole, which supports the 



