26 THE LEAF 



plunged in, it will be extinguished as quickly as in the 

 empty tumbler in the experiment described in Section 23, 

 thus showing that the process of respiration is more active 

 in this case than the opposite function of taking in carbon 

 dioxide and liberating oxygen. Insert a thermometer bulb 

 and note the difference in temperature. In some of the 

 arums, — -calla lily, Jack-in-the-pulpit, elephant's ^^zx {Colo- 

 casta), etc., — where a large number of small flowers are 

 brought together within the protecting spathe, the rise 

 of temperature is sometimes so marked that it may be 

 perceived by placing a flower against the cheek.^ 



30. Metabolism. — The total of all the life processes of 

 plants, including growth, waste, repair, etc., is summed 

 up by botanists under the general term Metabolism. It is 

 a constructive or building-up process when it results in 

 the making of new tissues out of the food absorbed from 

 the earth and air, and 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, a part being decomposed and excreted as 

 waste. In this sense, metabolism is said to be destructive, 

 and, like other destructive processes (combustion, for in- 

 stance), is always accompanied by the liberation of energy, — 

 heat, as we have seen, being an invariable accompaniment. 

 The waste in healthy plants is always, of course, less than 

 the gain, and a large portion of the food material is in 

 all cases laid by as a reserve store. For this reason, 

 photosynthesis, which is a constructive process, is usually 

 more energetic than respiration, 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 abundant oxygen for respiration, since the food 

 material manufactured by it must be decomposed into the 



1 See Sachs, " Physiology of Plants." 



