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 
Ns sides of the growing points of stems and 
Fic. 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 
Fy Nea Dees 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 
