THE LEAF 
19 
diffusion of water from the leaf into this moist air is less rapid 
than it would be into dry air. When there is free movement of 
air around. the leaf, wind will replace the 

ducing a screen, may also reduce 
Fic. 61. Scale of 
Elaeagnus philip- 
pensis. (X 75) 
the heating effect of sunlight. 
Hairs are usually most numer- 
ous on the under, or stomata- 
bearing, surface of the leaf. While a dense 
covering of trichomes retards transpiration, the 

Fig. 63. Stinging hair 
of a nettle (Laportea 
meyeniana). (Central 
drawing x 150) 
hairs of many leaves appear 
to be too scattered to have 
any appreciable effect. 
A covering of hairs, by 
holding a layer of air near 
the leaf, may prevent water 
from reaching and clogging 
the stomata. When a leaf 
with such a coating of tri- 
chomes is submerged in 
water, the layer of air near 
the epidermis glistens with 
a silvery sheen. Although 
moist air with drier air. A dense layer of tri- 
chomes tends to reduce the movement of the 
air and so to lower the rate of transpiration. 
A great development of trichomes, by pro- 
Fig. 62. Bris- 
tlelike hair of 
cowitch (Mu- 
cuna pruriens). 
(x 30) 
submerged, the epidermis may remain dry 
for a considerable length of time. Even 
such short trichomes as those shown in 
Fig. 35 may be very effective in keeping 
water from the stomata. While trichomes 
may restrict the rate of transpiration, they 
not only do not interfere with the diffusion 
of carbon dioxide into the leaf but may 
actually assist in keeping open a passageway for this diffusion 
when the stomata might be clogged as the result of rain or dew. 
