WORK OF LEAVES: DIGESTION IO/ 



179. The leaf as the organ of assimilation. The formation 

 of carbohydrates, the proteids and some other substances, while 

 taking place in all green parts of plants, is predominant in the leaf 

 because of its adaptation % to the process of photosynthesis, and the 

 close relation of the formation of other products closely dependent 

 on the carbohydrates. The sulphates, phosphates, nitrates, etc., 

 absorbed by the roots meet the carbohydrates in the leaf and are 

 here assimilated. 



IV. DIGESTION.* 



180. Digestion in plants includes those processes in which 

 food products and food substances either in the plant or outside 

 of it are changed chemically into a condition in which they can be 

 assimilated into new life substance or plant structures, or trans- 

 ported to other parts of the plant, or absorbed from the outside 

 if they are not already in a condition to be absorbed (the action of 

 roots on insoluble solutions in soil, and the action of ferments on 

 insects in case of carnivorous plants, paragraph 38). Several 

 kinds of digestion are performed by leaves (digestion of insects 

 by carnivorous plants is one example). One special kind of 

 digestion occurring in the leaf is the digestion of starch. The 

 starch formed in the leaf cannot be used for food by the grow- 

 ing leaf nor can it be transported to other parts of the plant until 

 it is dissolved. The starch is dissolved, or digested, in the leaf 

 by a special substance, leaf diastase, which is formed by the leaf 

 for this purpose. It is active at night, so that the starch formed 

 in the leaf during the day is changed to a sugar at night, and then 

 flows to other parts of the plant where it is assimilated by growing 

 organs, or is stored as reserve starch in seeds, stems, roots, tubers, 

 etc. For a discussion of diastase, see paragraph 219. 



* See also Chapter XVI. 



