THE LEAF 17 
Glandular hairs. A great variety of plants have glandular 
hairs (Fig. 64). These secrete oil, resin, or mucilage. A typical 
glandular hair consists of a stalk and an enlarged terminal por- 
tion, which is the gland proper. This may be unicellular or 
multicellular. The strong odors of many plants are due to the 
excretions of glandular hairs. 
SPECIALIZED LEAVES 
The principal function of ordinary foliage leaves is photosyn- 
thesis. Besides the primary function of foliage leaves they also 
perform such functions as respiration and growth, which are 
common to all liv- 
ing plant parts. 
In_ addition we 
find that many 
leaves are spe- 
cially modified to 
perform functions 
which are not usu- 
ally the princi- 
pal ones of leaves, 
or are modified to 
carry on the prin- 
cipal function in 
an unusual man- 
ner. Such leaves 
may be called 
pecalized ee: Left, Philippine grapefruit (Citrus decumana) (x 3) ; 
AS examples of right, Quassia amara (X 4) 
leaves which per- 
form the primary function of leaves in an unusual manner we 
may mention such leaves as those of oranges and lemons, where 
the petiole has a bladelike structure and is especially fitted for 
photosynthesis (Fig. 65), and the stipules of the garden pea 
(Pisum sativum), which are bladelike and have the functions of 

Fig. 65. Leaves showing bladelike petioles 
