plant, reaching- into every part: into the tips of the roots as well 
as up into the leaves and throughout the stem; so that practically 
every living cell of the plant is bathed on one or more sides by 
air from a tiny air space. 
These air canals in all land plants and even in all such water 
plants as the water lily, with floating leaves, open into the outer 
air by means of tiny mouth-like openings, called stomata, 
found on all green parts of stems and leaves. The two 
lip-cells, or guard-cells, of each stoma keep opening and 
shutting as they become drier or as they take up more water 
again. In the water lily, the stomata mouths are on the upper 
sides only of the floating leaves; and the air they take in passes 
through large air tubes, plainly visible to the naked eye, which 
lead down the stem and into the roots. Even water plants which 
are completely submersed in water, such as the pond-weeds, 
possess comparatively huge canals in their stems and leaf-stalks 
for circulating their oxygen and other internal gases. 
While this system of irregularly branching air tubes and 
canals in plants may seem very simple compared with the com- 
plex system for supplying air to the tissues in higher animals, it 
is nevertheless a very efficient system, for an adequate supply of 
fresh air seems to be rarely lacking for plants; except perhaps in 
such situations as where a land plant like corn, e.g ., is drowned 
out with water. Such plants as alfalfa and corn cannot stand 
“wet feet’ ’ because the extra water about their roots prevents them 
from getting sufficient fresh air; so that they become literally 
smothered for lack of sufficient air. 
An elaborate circulatory system such as is seen in higher 
animals is thus seen to be unnecessary for carrying fresh air to 
the tissues in plants; as the stomata and the system of air tubes 
connected with them adequately perform this duty. In the 
higher animals, the blood vascular system has a double func- 
tion: to carry food as well as air. In plants, in addition to the 
air canals and air spaces, there is a sort of system of tubes for 
carrying the sap and food materials, but it is so simple that it 
can hardly be compared with the blood-carrying system of 
animals. 
Fresh air, then, is as necessary for plants as it is for 
animals. Respiration, in its essential features: the taking in of 
