176 PRACTICAL COURSE IN BOTANY 
life processes by which it is used up. The difference between 
them will be made clear by a comparison of the two pro- 
cesses as summarized in the following statement: 
PHOTOSYNTHESIS RESPIRATION 
Goes on only in sunlight and in —_ Goes on at all times and in all 
the green parts of plants. parts of the plant. 
Produces starch and sugar. Releases energy (heat and work- 
ing power). 
Gives off, as by-product, oxygen. Gives off, as by-products, COs, 
and water. 
A constructive process, in which A destructive, or consumptive 
energy is used up to make food. process, in which food is used up in 
expending energy. 
193. Metabolism. — The total of all the life processes of 
plants, including growth, waste, repair, etc., is summed up 
under the general term metabolism. It is a constructive or 
building-up process when it results in the making of new 
tissues out of food material absorbed from the earth and air, 
and the consequent increase of the plant in size or numbers. 
But, as in the case of animals, so with plants, not all the 
food provided is converted into new tissue, part being used 
as a source of energy, and part decomposed and excreted 
as waste. In this sense, metabolism is said to be destructive. 
The waste in healthy growing plants is always, of course, less 
than the gain, and a portion of the food material is laid by 
as a reserve store. For this reason, photosynthesis, being a 
constructive process, is usually more energetic than respira- 
tion, which is the measure of the destructive change of 
materials that attends all life processes. 
It is evident also, from what has been said, that growth and 
repair of tissues can take place only so long as the plant has 
sufficient oxygen for respiration, since the energy liberated 
by it is necessary for the assimilation of nourishment by 
the tissues. 
Thus we see that plants are dependent on air not only for 
respiration, but for nutrition, and none of their life pro- 
cesses can be carried on without it, 
