PHYSIOLOGY OF THE COTTON PLANT 23 



The dry matter is composed largely of combustible 

 material, nearly all of which comes from the air and water. 

 Four elements enter into the composition of the combusti- 

 ble part. These are carbon, hydrogen, oxygen, and nitro- 

 gen. The ash which is left after the dry matter has been 

 burned, is composed of mineral matter taken from the 

 soil. Less than 2 per cent of the weight of a green cotton 

 plant is secured from the soil. 



24. The essential constituents. There are ten ele- 

 ments essential to plant growth. Of these ten elements, 

 four are metals and six are non-metals. The four metals 

 are potassium, calcium, magnesium, and iron, all of which 

 the plant secures directly from the soil. Of the non- 

 metals, two, sulfur and phosphorus, are secured directly 

 from the soil, while nitrogen is obtained indirectly from 

 the air through the soil. The remaining three are carbon, 

 obtained largely from the carbon dioxide of the air, and 

 hydrogen and oxygen, obtained from water (some hydro- 

 gen is obtained from ammonia and some oxygen from the 

 air). Those elements that are derived from the soil are 

 absorbed in the form of salts. 



NUTRITION 



The growing cotton plant is dependent upon certain 

 vital activities for its existence, such as the absorption 

 of food and water, the assimilation of carbon dioxide, the 

 digestion of the raw food materials, the giving off of oxy- 

 gen and water, and the securing of the necessary energy 

 for these activities. The processes which promote growth 

 and repair the waste caused by the vital activities are 

 called nutritive processes. 



25. The absorption of food. The essential food 

 elements were discussed in paragraph 24. The structure 



