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 wo.k- 



ing power). 

 Gives off, as by-product, oxygen. Gives off, as by-products, CO3 



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. 



