258 HOW CROPS GROW. 
are fringed with a profusion of delicate hairs, and rapidly extend toa 
iength of from one to two inches. The lower ones, if they chance to 
penetrate the water, become discolored and decay; the others, howevet,- 
vemain for a long time fresh, and of a white color. 
As already mentioned, Indian corn frequently produces 
air-roots. The same is true of the oat, of buckwheat, of 
the grape-vine, and of other plants of temperate re- 
gions when they are placed for some time in tropical con- 
ditions, i. e., when they grow in a rich soil and their over- 
ground organs are surrounded by a very warm and very 
moist atmosphere. 
It has been conjectured that these air-roots serve to ab- 
sorb moisture from the air and thus aid to maintain the 
growth of the plant. This subject has been studied by 
Unger, Chatin, and Duchartre. The observers first named 
were led to conclude that these organs do absorb water 
from the air. Duchartre, however, denies their absorptive 
power. It is probably true that they can and do absorb 
to some extent the water that exists as vapor in the at- 
mosphere. At the same time they may not usually con- 
dense enough to make good the loss that takes place in 
other parts of the plant by evaporation. Hence the re- 
sults of Duchartre, which were obtained on the entire 
plant and not on the air-roots alone. (Hléments de 
Botanique, p. 216.) It certainly appears improbable that 
organs which only develope themselves in a humid atmos- 
phere, where the plant can have no lack of water, should 
be specially charged with the office of collecting moisture 
from the air. 
Root-Excretions.—It has been supposed that the roots 
of plants perform a function of excretion, the reverse of 
absorption—that plants, like animals, reject matters which 
are no longer of use in their organism, and that the re- 
jected matters are poisonous to the kind of vegetation 
from which they originated. De Candolle, an eminent 
French botanist, who first advanced this doctrine, founded 
