FUNCTIONS OF EPIDERMAL TISSUE. 787 
separates out in the form of drops. This wax may be associated 
with resin, and assists in preventing the aerial parts of plants 
from becoming moistened by water. Such plants as these are 
best fitted for growth in houses, where the air is usually very 
dry. While the epidermis may thus be shown to have for its 
object the restraining of a too abundant exhalation, the stomata 
and water-pores are especially designed to facilitate and regulate 
the passage of fluid matters, and in proportion to their number, 
therefore, upon the different organs and parts of plants, ceteris 
paribus, so will be the exbalation from them. (See also page 63.) 
Stomata, as already noticed (page 62), are sometimes found at 
the bottom of depressions on the under surface of leaves (fig. 124), 
and occasionally projecting above the general level, but usually 
they are placed nearly or quite on a level with the epidermal 
cells. The exact manner in which the stomata act is not readily 
explained, but it may be always noticed that when plants are 
freely supplied with moisture, the stomata have their bordering 
guard-cells distended with fluid, elongated, and curved, so that 
the orifices between them are open ; whilst in those cases where 
there is a deficiency of fluid the bordering cells contract, 
straighten on their inner surfaces, and thus close the orifices. 
Under the former condition of stomata, there is a ready com- 
munication between the external air and the internal tissues, 
and hence a free exhalation takes place; while in the latter 
state, the exhalation is more or less prevented. As a rule, sto- 
mata are open during the day when circumstancesare favourable, 
and closed at night when the plant is asleep. 
It is also through the cells of the epidermis, and more espe- 
cially through the stomata, that certain gaseous matters are 
absorbed from, and exhaled into, the atmosphere, in the pro- 
cesses of Respiration and Assimilation. (See page 799.) 
Tt has long been a disputed question whether the epidermal 
tissue and its appendages have the power of absorbing liquids, 
such as water. Some authors, as Unger and Duchartre, not 
only deny the possession of such a power, but also that of taking 
up watery vapour; and Prillieux has repeated their experi- 
ments with the same results and conclusions. Some researches: 
of Henslow seem, however, to prove that leaves can absorb 
moisture. (See page 799.) Indeed, it is very difficult to ac- 
count for the immediate recovery of drooping plants in a green- 
house when water is sprinkled upon the floors, or the revival in 
nature of vegetation when a mist follows a long succession of 
dry weather, except on the supposition that watery vapour is 
taken up by the epidermal tissue and its appendages, unless the 
presence of moisture acts only in the way of checking transpira- 
tion. Epiphytical species seem also to obtain nourishment from 
the atmosphere by absorption through the epidermis. Whether 
water itself is absorbed by the epidermal tissue and its appen- 
dages is doubtful, though from the experiments of Detmer it 
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