THE MATERIAL INCOME OF PLANTS 
321 
stomata, and on the under side of the olive leaf about 3700. The 
following table (after Weiss) shows the numbers per square millimeter 
in various common plants. 
NAME OF PLANT 
NUMBER OF 
STOMATA 
NAME OF PLANT 
NUMBER OF 
STOMATA 
Upper 
side 
Under 
side 
Upper 
side 
Under 
side 
Olea europaea (olive) . . 
Castalia cdorata (white 
water lily) 
O 
460 
175 
O 
60 
IOI 
O 
625 
O 
325 
33 
263 
216 
I4S 
Zea Mays (Indian corn) . 
Betula alba (white birch) 
Berberis vulgaris (bar- 
berry) . 
94 
o 
O 
89 
142 
48 
o 
158 
237 
229 
131 
o 
27 
62 
Helianthus annuus (sun- 
flower) 
Populus deltoides (cot- 
tonwood) 
Syringa vulgaris (lilac) 
Solanum Dulcamara (bit- 
tersweet) 
Pinus Strobus (white 
pine) 
Pisum sativum (pea) . . 
Ficus elastica (rubber 
plant) 
Avena sativa (oats) . . 
Lilium bulbiferum (tiger 
lily) 
So far as plants have been examined, it appears that a large majority 
of mesophytes have less than 200 stomata to the square millimeter, 
and a fair average is perhaps 150. (See Part III, p. 556, on variations 
in the structure and distribution of stomata, and the causes thereof.) 
Transpiration. Since the intercellular spaces are bounded by moist 
cell walls, freely permeable to water, they are always filled with air 
which contains more or less water vapor. This vapor diffuses through 
the stomata into the drier outer air, and being lost from the plant will 
be replaced in whole or in part by water entering the root. At the 
same time, since the walls of the epidermal cells contain a little water, 
some evaporation takes place directly from them. The total evapora- 
tion of water under these conditions is designated as transpiration 
(see p. 323). 
Exit but no entry for water. The aerial parts are constantly losing 
water because they are permeable ; at the same time, there is practically 
no opportunity for the admission of water, even when such parts are 
deluged by it. Ordinarily rain comes into contact only with a nearly 
waterproof surface, the cuticle. It cannot easily penetrate the minute 
stomata, even when they occur on the upper surface of leaves, for there 
