CHAPTER VIII 

 THE ASSIMILATION OF PLANT FOOD 



58. Source of Plant Food. The plant derives its food 

 from two sources the atmosphere and the soil. By far 

 the greater portion is obtained from the atmosphere. Of 

 the ten elements necessary for plant growth carbon, hy- 

 drogen, oxygen, nitrogen, phosphorus, potassium, sulphur, 

 iron, calcium, and magnesium carbon, hydrogen, and 

 oxygen compose about 95 per cent of all agricultural crops. 

 These three elements are supplied from the atmosphere. 

 Some of the nitrogen used by plants is also derived indi- 

 rectly from the atmosphere, from which it is " fixed " by 

 means of bacteria (see page 38). The soil elements nec- 

 essary for plant growth are taken up by the plants from 

 water solution. The root system of the plant constitutes 

 the channels for taking up the solutions. 



According to their root systems, plants may be divided 

 into two main classes, those that have tap roots and those 

 that haoe a mass of lateral, fbrous roots. The tap-rooted 

 plants usually penetrate deeper into the soil. The main 

 roots of the former class have smaller branches, and these, 

 in turn, are covered with root hairs (Fig. 24) which are cells 

 that act like syphons. They consist of cells which contain 

 granular protoplasm and sap. These root hairs are widely 

 distributed throughout the soil, and come in intimate con- 

 tact with the soil particles. The soil particles are covered 

 with capillary moisture containing certain amounts of 

 plant food which has been dissolved from the particles. 

 This moisture with its dissolved material is taken up by 

 the root hairs and conveyed by osmotic pressure into the 



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