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 
