PHYSIOLOGY 475 



ity of these succulents is associated with the small development of 

 the stomata, and the consequent difficulty of procuring a sufficient 

 supply of C0 2 and perhaps free oxygen from the atmosphere. 



In some instances (Spirogyra, leaves of Phaseolus), where the 

 green cells are deprived of C0 2 , or kept in darkness, they are capable, 

 when they are placed in a solution of dextrose, of manufacturing 

 starch, but they soon die unless the normal conditions are supplied. 

 Lemna, also, can be grown for a time in darkness upon a sugar or 

 glycerine solution. 



External Conditions for Photosynthesis 



Light is not only necessary for photosynthesis, but in most in- 

 stances for the development of chlorophyll as well. There are 

 numerous exceptions to the latter statement, however, as chlorophyll 

 develops in darkness in the embryo of many Seed-plants (Coniferse, 

 Celastrus) and in the leaves of Ferns and the prothallium and 

 embryo of Pilularia. The same rays, i.e. those between the Frauen- 

 hofer lines B-D of the spectrum, which are most effective in C0. r 

 assimilation, are also most active in causing the development of 

 chlorophyll. Chlorophyll, moreover, does not form below a certain 

 temperature. The blanched appearance of Indian-corn, sprouted 

 in cold weather, is an illustration of this point. 



The amount of oxygen, warmth, the amount of CO 2 , and, in short, 

 the conditions which in general affect the vital processes of the 

 plant, all influence the activity of photosynthesis. 



Chemosynthetic Assimilation of C0 2 



Until very recently it has been assumed that the assimilation of 

 carbon-dioxide was entirely confined to cells containing chlorophyll, 

 through energy derived from light. It is now known that the 

 nitrogen Bacteria can assimilate C0 2 , by virtue of energy derived 

 from the oxidation of ammonia into nitrites, and of nitrites into 

 nitrates. The whole of the organic food of these organisms is 

 obtained in this way, and they can develop in a fluid free from all 

 organic matter, provided the necessary inorganic salts are present. 

 Sulphur Bacteria obtain energy for CCVassimilation from oxidation 

 of H 2 S. 



Assimilation of Organic Food 



Plants which are unable to assimilate C0 2 must obtain their car- 

 bonaceous food from organic compounds. Such plants are either 

 parasites or saprophytes, as they obtain their food from living or 

 dead organisms. Pfeffer calls all parasites, as well as organisms 

 which live together without injury to either, Symbionts. In the 



