152 CONSTRUCTION OF PLANT'S FOOD 



losing too much water, rolls up from both edges, so that the free 

 margins of the incomplete epidermis touch or overlap. Here as 

 in Marchantia the products of photosynthesis are given over to 

 elongated conducting cells in the lower part of the leaf. 



In many mosses the blade of the leaf on either side of the 

 midrib is one cell thick and all of these cells contain chloro- 

 plasts, and being elongated parallel with the long axis of the 

 leaf they conduct away their own products (Fig. 86). 



Synthesis of Food Without Light. — Some kinds of bac- 

 teria (Nitrococcus and Nitrosomonas) inhabiting the soil oxi- 

 dize ammonia and its compounds to nitrous acid and its salts 



Pig. Ss. — Cross section through a portion of leaf of Polytrichunx commune, b, chains of 

 chlorophyll-bearing cells; d, bast fibers. (After Strasburger.) 



and utilize the kinetic energy liberated by this oxidation in 

 making food from carbon dioxide and water. Other bacteria 

 included in the genus Nitrobacter oxidize the nitrous to nitric 

 compounds and get energy in this way for food synthesis. Still 

 other bacteria utilize the energy from the oxidation of salts of 

 sulphur and iron occurring in the soil. 



In green plants the construction of nitrogenous foods by unit- 

 ing carbohydrate made in the leaves with salts of nitrogen, and 

 sometimes in addition, with salts of sulphur and phosphorus 

 brought up from the soil, can take place in darkness as well as 

 in the light, and possibly in any living cell. When proteid syn- 

 thesis goes on in darkness the energy for the work apparently 

 comes from the decomposition of a part of the carbohydrates, 

 in which event sunlight would still be, although indirectly, the 

 source of the energy. 



