36 



PRINCIPLES OF PLANT CULTURE 



Food manufacture (photosynthesis) and tissue building 

 (assimilation) are not necessarily simultaneous, but either 

 may proceed independently of the other. 



Green plants can manufacture their food from mineral 

 substances. The food of animals must all have been first 

 formed by plants. Green plants differ fundamentally 

 from animals in this respect. These plants, as just ex- 

 plained, can with the help of energy absorbed from sun- 

 light manufacture their food from mineral substances. 

 The food of animals on the other hand must be obtained, 

 directly or indirectly, from green 

 plants. 



59. The sources of plant-food. — 

 By observing plantlets of the bean 

 or pumpkin a few days after germi- 

 nation, we may discover that the 

 cotyledons, which were at first so 

 plump, have shriveled to a mere frac- 

 tion of their former size. They have 

 shriveled because the food contained 

 by these parts has been absorbed 

 by the developing plantlet. The 

 patrimony furnished by the seed is 

 Whence then comes the food that is 

 to complete the development of the plant? Aside from 

 the carbonic acid already mentioned (58), several other 

 substances are required to build up the plant structure. 

 These are almost wholly derived from the soil, through 

 the medium of the water absorbed by the root-hairs 

 (100). They must all be dissolved in the soil water or 

 they cannot enter the plant, for they must pass through 

 the cell-walls, which are not permeable to undissolved 

 solid matter. 



Fig. 16. — Grains of 

 starch stored as re- 

 serve food in a cell 

 of potato. Highly 

 magnified. 



quickly exhausted. 



