Chemical Processes in Plants. 323 



and dextrose is probably formed as an intermediary pro- 

 duct 



6CO a + 6ELO = C 6 H 12 6 + 60 2 



The gaseous exchange in the leaf takes place through 

 openings on the surface (which in most plants are chiefly 

 on the under-surface), which are known as the stomata. 

 Starch formation takes place with great rapidity after 

 exposure to light, and can be detected within a few 

 minutes. The exact mechanism of starch production is 

 unknown.* It has been suggested by Bayer that the first 

 assimilation product of the carbon dioxide is formaldehyde. 

 It is known that this substance in the presence of alkalis 

 readily polymerizes to form hexoses 



6CH 2 = C 6 H 18 0, 



It has been shown by Fenton that carbon dioxide can 

 be reduced to formaldehyde by amalgamated magnesium, 

 and other methods of reducing it to the aldehyde have 

 also been described. Formaldehyde, furthermore, can be 

 detected in the growing plant. It has not been proved, 

 however, that this arises from the reduction of carbon 

 dioxide, for thin films of chlorophyll, when exposed to sun- 

 light, even in the absence of this gas, also give rise to the 

 aldehyde, which is a product of photochemical decomposition 

 of a large number of organic substances. Nevertheless, 

 the formation of formaldehyde from carbon dioxide as an 

 intermediary product of assimilation to starch is not at all 

 improbable ; in fact, it has been shown that plants can 

 produce starch when grown in very weak solutions of 

 formaldehyde (which in stronger solutions is highly toxic) 

 or in solutions of the less toxic bisulphite compound. 



* A recent monograph by Willstatter and Stoll on carbon assimilation has 

 appeared which was not available to the author at the time of this book going 

 to press. 



21 a 



