CHAPTER IV 
THE LEAF 
A leaf is a lateral outgrowth from a stem and is, typically, a 
thin, expanded structure with, usually, a green color (Fig. 15). 
While a number of important physiological processes take 
place in leaves, their principal function, and the one for which 

Fig. 15. Young leaves of mulberry 
(Morus alba) 
Stipules occur at the bases of all except 
the oldest ; here there is a scar showing 
where the stipule was located. (x 4) 
their structure is especially 
suited, is photosynthesis. This 
is the production of sugar 
from carbon dioxide and 
water in the presence of 
sunlight. This process takes 
place only where there is 
chlorophyll, which occurs in 
the chloroplastids. The sugar 
formed in photosynthesis is 
the basis from which all the 
complex compounds found 
in plants are produced. The 
thin, expanded form of leaves 
is especially suited for pho- 
tosynthesis, as light, which 
is necessary for this process, 
penetrates only a short distance into a plant. An increase in the 
thickness of leaves would require additional plant material without 
producing a corresponding increase in the rate of photosynthesis. 
Structure of leaves. Leaves (Fig. 15) are typically composed 
of a broad, expanded portion, the blade, which is the essential 
part; a stalk, called the petiole, which is sometimes lacking ; the 
base, the part which joins the leaf to the stem; and often a pair 
26 
