THE LEAF 251 
the Myrtaceae. The leaflets of Cassia Senna and Cassia angustifolia 
which constitute the drug Senna, show a marked bifacial struc- 
ture. Both surfaces are similar, having stomata about equal in 
number. ‘The mesophyll (chlorenchyma) is differentiated into a 
central spongy parenchyma containing bundles, and a zone of 
palisade cells on either side facing the epidermises (see F ig. 170). 
STRUCTURE AND DEVELOPMENT OF STOMATA.—Stomata or 
stomates are slit-like openings in the epidermis of leaves or young 
green stems surrounded by a pair of cells, called guard cells, 
whose sides opposite one another are concave. They form a 
Fic. 169.—Portion of a transverse section of the White Pine leaf showing 
xerophytic-centric structure. ¢p, Epidermis; s, stoma; gc, guard cells; s’, sub- 
stomal air-space; h, hypodermis, consisting of strengthening fibers which form a 
scleroid band beneath epidermis; f, infolded parenchyma cell of mesophyll. 
Greatly enlarged. 
communication between the intercellular-air-space (respiratory 
cavity) beneath them and the exterior. The slit-like openings, 
taken with the guard cells, constitutes what is known as the 
stomatal apparatus. The stomatal apparatus is frequently called 
in practice, ‘‘stoma” or “‘stomate.” 
The epidermal cells which abut on the stomatal apparatus are 
called neighboring cells or subsidiary cells. "These, in many cases, as 
in species of Helleborus, Sambucus, Hyacinthus, Peonia, Ferns, etc., 
are very similar to the other epidermal cells, but in a large num- 
ber of plants they differ in size, arrangement and shape from the 
other cells of the epidermis which do not abut upon the stomatal 
apparatus. In Senna they are mostly two in number, one larger 
than the other and arranged parallel to the guard cells of the 
