170 Chapters in Modern Botany CHAP. 



all forms of respiration, and furthermore, that the leaves 

 are the organs by which the breathing is principally 

 effected. 



It is difficult to exorcise a persistent fallacy which has 

 long plagued the student, and for which tradition and text- 

 books are much to be blamed that one of the great 

 differences between plants and animals is that plants take 

 in carbonic acid gas and give out oxygen, while animals 

 take in oxygen and give out carbonic acid gas. This is a 

 fallacy, all the more troublesome because both its statements 

 are true. It is true that animals take in oxygen and give 

 out carbonic acid gas this is the essence of respiration. 

 It is also true that green plants in sunlight take in 

 carbonic acid gas and give out oxygen this is one of 

 their essential characteristics. But it is also true that 

 plants, like animals, take in oxygen and give out carbonic 

 acid gas, for they must breathe. Only it happens that 

 in the daytime the process of respiration in green plants 

 is externally hidden, is virtually counteracted (since much 

 more than compensated) by the reverse nutritive process 

 peculiar to plants. The fallacy is the contrast between a 

 respiratory process, common to all creatures, and readily 

 demonstrable in plants which are not green, or which are 

 in darkness, and a nutritive process which is peculiar to 

 the green parts of plants during the light of day. 



Summary of Leaf Functions. We are now in a 

 position to sum up the chief functions of leaves : 



(1) Leaves are the principal organs by means of which 

 the plants give off surplus water, and that chiefly on the 

 under surface and through the minute apertures or stomata 

 which regulate the transpiration. 



(2) Leaves are the principal organs by means of which 

 the plants breathe, by which the gaseous interchange which 

 is implied in all life is effected. But during the day in 



