596 
PRESIDENTS ADDRESS. 
which are cast off as waste products, such as various alkaloids, 
acids, gums, resins, &c. Others however, are useful in the general 
economy of the plant, such as nectar for the purpose of attracting 
insects. In order that a plant may be vigorous and strong it must 
obtain energy to enable it to carry on the various functions of cell 
growth, secretions, &c. This can only be supplied by oxygen. 
There is never a state of equilibrium in a plant, but metabolic 
processes are constantly at work. Atoms of protoplasm are always 
breaking down, and as often being built up again. The carbo- 
hydrates and proteids of the formed material are also constantly 
disintegrating and new combinations being formed. 
All these actions are brought about by the assimilation of oxygen 
and the setting free of carbon dioxide, which on account of its 
poisonous nature is thrown off by the plant and escapes at various 
points, by the stomata of the leaves, the flowers, and probably by 
the roots. These metabolic actions, in which oxygen is assimi- 
lated, are termed respiration, and are in constant operation 
throughout the whole plant ; but inasmuch as the chloroplasts are 
inactive in the absence of light, the exhalation of carbon dioxide 
is more manifested at night. On the return of light, when the 
chloroplasts again become active, a certain amount of the respired 
carbon dioxide is utilized again in photosynthesis. We must 
remember that these chemical processes that go on in the plant are 
not simply physical, but they have their origin in the living pro- 
toplasm, and are vital and physiological. If the protoplasm is not 
supplied with oxygen, none of these metabolic processes take 
place, and the plant ceases to live. 
A plant, like an animal, cannot live without water and plenty of 
it. It is essential for perfect health in a growing plant that all its 
living cells should always be in a turgid condition, in order that 
dissolved food material may freely diosmose from cell to cell, and 
this can only be ensured by a constant supply of water from the 
roots. The liquid taken up by the roots is pure water, holding in 
solution certain mineral salts derived from the soil, and it is ab- 
sorbed by the root, hairs and cuticular cells of the terminal growing 
rootlets ; it is forced under root pressure into the wood tissue, and 
