LEAVES IN SEASONS 151 



drogen and oxygen in six molecules of 

 formic aldehyde are rearranged in one com- 

 plex molecule forming sugar, from which the 

 other carbohydrates are easily derived. Pro- 

 toplasm may not be formed from sugar alone, 

 since nitrogen is a very important constituent 

 of living substance. It is probable that nitro- 

 genous substances are sometimes formed by a 

 variation in the earlier stages of the process 

 described above, by which nitrogen is substi- 

 tuted for oxygen in the molecule of formic 

 aldehyde. Such a substitution would result 

 in the formation of hydrocyanic acid. The 

 recent discovery of this deadly acid in the 

 leaves of a tropical palm lends favor to the 

 hypothesis. It may be formed in many green 

 leaves, but, like the earlier substances in the 

 synthesis of sugar may undergo instant trans- 

 formation and thus escape detection. 



The absorption of carbon dioxide from the 

 air, and the excretion of oxygen by vegeta- 

 tion is sufficient to balance the opposite 

 process in animals, and hence the composition 

 of the atmosphere remains unchanged. It is 

 a notable fact that plants thrive in an atmo- 

 sphere containing a much larger proportion 

 of carbon dioxide than is found in the atmo- 

 sphere at the present time. Normal air con- 

 tains but one-twenty -fifth of one per cent of 

 this gas, and the food-forming power of the 



