270 LEAVES 



compounds are broken into their constituents, usually through 

 oxidation, with the result that the latent energy is released to be 

 used in various kinds of work. Second', photosynthesis uses 

 carbon dioxide and releases oxygen, while respiration uses oxygen 

 and releases carbon dioxide. Hence one liberates the gas which 

 the other uses, and in this way the two processes tend to sup- 

 port each other. When both processes are active at the same 

 time, as during the day, each process tends to obscure the other 

 by using the gases liberated before these gases escape from the 

 leaf. However, when photosynthesis is active, the amount of 

 carbon dioxide used and oxygen liberated is so much greater than 

 the gaseous exchanges of respiration that the latter process is 

 entirely obscured. On this account, botanists once thought that 

 respiration was a process performed only by animals and that the 

 plant breathes in a way just opposite from that of animals. Of 

 course further investigations showed that plants respire just the 

 same as animals do, but in addition green plants carry on photo- 

 synthesis which, when active, so much obscures respiration that 

 the latter process had escaped notice. Third, photosynthesis 

 depends upon the presence of light, while respiration is inde- 

 pendent of light, being active at night as well as in the daytime. 

 At night when there is no photosynthesis to obscure respiration, 

 plants take in oxygen and liberate carbon dioxide just as animals 

 do, and the notion once prevalent that plants purify the air is 

 only true of them when they are engaged in photosynthesis. 



Respiration and transpiration, although influencing each other 

 to some extent, are also distinct processes. Since respiration 

 liberates energy, some of which is in the form of heat, respiration 

 may increase transpiration by raising the temperature of the leaf. 

 Furthermore, respiration in breaking down compounds releases 

 water in the form of vapor, in which form it readily escapes to the 

 air. On the other hand, when transpiration reduces the water 

 content of cells so much as to interfere with the activities of the 

 protoplasm, then respiration may be retarded. 



Special Forms of Leaves 



In contrast to the leaves which we have been studying, there 

 are some leaves which have become so modified as to resemble 

 ordinary leaves very little. Some have become so changed that 



