PRESIDENTIAL ADDRESSES. 31 
tropics, they are likely to be subjected to low temperature, to which they must 
become adapted in order to live. No plant can vegetate at a temperature below 
freezing, but by proper adaptation many plants can endure an indefinitely low 
temperature in the resting stage. In high latitudes it becomes a question of 
ability to endure the long winter. Plants have been found as far north as soil 
conditions have permitted their growth. At high altitudes, however, vegetation 
is limited by the lack of a growing season. The temperature may remain through- 
out the year below the point at which growth is possible. Under this head, then, 
is the study of the methods of resisting injury from low temperature. 
Liceut.— Too intense light is injurious, while with too little light the process 
of photosynthesis cannot be carried on. By assuming the profile position, either 
temporarily, as in motile leaves, or permanently, as in compass-plants, with verti- 
cal leaves, injury from too intense light can be avoided. Others must exert them- 
selves to obtain the maximum amount of light. Some plants bring the leaves 
into the maximum light position by heliotropic movements. Submerged water- 
plants and the shade plants of dense forests are obliged to adapt themselves to 
the minimum amount of light for growth. 
Air.—Aside from the indirect effect upon transpiration, the air relations of 
plants may be divided into about three groups: (1) The supply of oxygen for the 
purpose of respiration, which is necessary to all active cells. Interesting adapta- 
tions are found in swamp plants for leading air down to the roots: aerating tis- 
sues, air canals, cypress knees, etc. (2) The direct mechanical effect of wind 
upon the plant body. (3) The methods by which plants utilize air-currents for 
the dispersal of seeds and pollen. 
Sorz.— Soil relations may be chemical or physical, the latter being by far the 
most important. The elements necessary for plant growth are usually present in 
cultivated soil in sufficient amount, and, usually, there is nothing in the soil 
which is of direct injury. The physical condition of the soil is chiefly concerned 
with the water relation. When the soil contains a large amount of soluble salts 
plants have difficulty in absorbing sufficient water; and when the soil water is as 
concentrated as the cell sap, plants cannot exist. Such conditions obtain in salt 
marshes, alkali plains, etc. And the plants are of a pronounced xerophytic type, 
from the necessity of reducing transpiration to suit water-supply. 
AnimaLs.—Animals use plants for food, and they inflict injury by tramping 
upon them. Hence, protective devices in the shape of thorns and prickles, or a 
bitter or irritating sap, and the rosette habit. Insects aid in the pollination of 
flowers; a study of which is an important branch of ecology. Animals aid in 
the dispersal of seeds, by accidentally carrying those which have devices for 
attachment, or by eating the fruits, in which the large seeds are thrown away and 
the small ones escape digestion. Special relations which are extremely inter- 
esting occur between ants and plants. Certain leaf-cutting species of ants culti- 
vate a kind of fungus in their underground nests. The leaves are gathered to 
furnish a substratum for the fungus, which, in its turn, yields a peculiar fructi- 
fication upon which the ants feed. Other species, enemies to the preceding, are 
retained as guards by several kinds of plants, which thus escape denudation. 
These plants furnish especially formed homes, and often also food for their pro- 
tectors. The homes and special-food bodies appear to have no other function 
than that of inducing the guards to take up their abode there and recompensing 
them for their trouble. 
OTHER Puiants.— Every individual, from the time it is cast loose from the 
mother plant as a seed, must struggle against the various factors mentioned, but. 
