LEAF STRUCTURE AND DEVELOPMENT 35 
palisade cells. The remainder of the space is filled with a mass of 
rather loose, spongy parenchyma, in which the smaller veins are im- 
bedded. The veins are 
composed of a few spiral 
ducts and tracheids 
(TEXT FIG. 37, B) near 
the upper epidermis, 
from which they are 
separated by three or 
four layers of long, thin- 
walled non chlorophyll- 
bearing cells (TEXT FIG. 
37, A) and a small irreg- 
ular group of phloem 
elements below (TEXT 
FIG. 37, C). The stif- 
fening factor is a prom- 
inent strand of collen- 
chyma (TEXT FIG. 37, D) 
making up the greater 
part of the ridge on the 
> 
U 
AAY OTITO 
38 
Fic. 37. Cross section of a mature leaf 
under side of the leaf. 
In surface view the 
lower epidermis shows 
A cross section of a young leaf 
with the first signs of differentiation of cells 
indicating the position of vascular strands, A. 
irregular cells more or 
less interlocked by means of undulating walls (TEXT FIG. 39). 
The stomata average 50 to the sq. mm. and show an average ex- 
treme width of 28 microns and an average extreme length of 40 
Adjoining the guard cells is a pair of accessory cells 
(TEXT FIG. 39, A). These accessory cells are sisters of their con- 
tiguous guard cells and are formed by a second division of the initial 
cell. Occasional twin stomata (TEXT FIG. 41) have been observed, 
which have probably resulted from a division of the cells which 
usually form guard cells. The upper epidermis is composed of 
cells with much more regular outline, and usually shows no 
stomata (TEXT FIG. 40). 
As the blade appears in the differentiation of the primordium 
it is composed of five layers of similar cells (TEXT FIG. 38). The 
