WORK OF LEAVES: PHOTOSYNTJlttS T . IOL 



one, or rather suffocating gas. We have 'ajr^ad^ . no&ed ' iii*tf 

 carbon dioxide is easily absorbed by water. Water is a com- 

 pound of two gases, hydrogen two parts and oxygen one part, 

 and the symbol for water 

 is therefore H 2 O. The 

 land plants absorb* some 

 of the carbon dioxide from 

 the air, and it is also Fig. 87. 



- , . , . , The splinter lights again in the presence of 



formed within the plant oxygen gas . 



cells of all plants during respiration. But whether formed in 



the plant cell or absorbed from the air, as soon as it comes 



in contact with the water in the cell it is absorbed. When 



carbon dioxide is absorbed by water, the two together form 



carbonic acid (CO 2 + H 2 O = CH 2 O 3 = the symbol for carbonic 



acid).f 



169. There are five principal requirements in the process 

 of photosynthesis! during which the formation of starch and 

 sugar in green plants takes place. First, the living plant cell; 

 second, the presence of carbonic acid; third, the presence of chloro- 



* The process of transpiration keeps the cell wall saturated with water. 

 This provides the water for the solution of the CO 2 which in the dry con- 

 dition (anhydride condition) could not diffuse through a dry cell wall fast 

 enough. This is believed to be one of the important functions of trans- 

 piration. In the case of aquatic plants the CO 2 in the water is dissolved 

 and is therefore CH 2 O 3 = carbonic acid. 



t In the case of aquatic plants the water in which they grow absorbs 

 carbon dioxide from the air, thus forming the carbonic acid which they 

 absorb from the water. 



I It is now evident what photosynthesis means. The first part of the 

 word comes from the Greek word 0orr6s = light, and the last part of the 

 word has reference to the synthetic process, or union, or putting together. 

 The process was formerly considered a process of assimilation and was 

 called carbon dioxide assimilation. True assimilation, however, is brought 

 about entirely by the living substance of the plant (see paragraph 176). 

 In "carbon dioxide assimilation" sunlight supplies the energy for one step 

 in the process, and it seems better to use the term photosynthesis, though 

 if the nature of the process is kept clearly in mind there should be no very 

 great objection to saying carbon dioxide assimilation. Some use the term 

 fixation of carbon. 



