DESCRIPTION OF PLATES. 173 
from the different forms and various modes of 
grouping of these epidermal cells, and of the 
beautiful arrangement, for the end in view, of 
the cells of the stomata, or breathing-pores. 
These stomata are openings through which air is 
admitted to the lungs (leaves) of the plant. 
On each side of the stoma, in most plants, there 
is a semilunar “ guard-cell.” These are well seen 
in Fig. 1. The office of this pair of cells seems 
to be to regulate the admission of air through 
the stoma according to the requirements of the 
plant, and with reference to the hygrometric 
condition of the atmosphere. Evidently the sto- 
mata (apertures ) will be of greater or less size 
according as the guard-cells are distended or oth- 
erwise with fluid. The stomata are mostly found 
upon the lower surface of leaves, although in 
some species they are also abundant upon the 
upper surface as well. 
Their number has been estimated, by careful 
counting, for a number of species, and is given 
as 160,000 per square inch in the lilac; 63,000 
in the holly; 2,000 in Tradescantia; 200 in the 
mistletoe, etc. The reader interested in vege- 
table histology and physiology, will do well to 
obtain a copy of Professor Sachs’ “Text-book of 
Botany” (English translation by Bennett). 
