24 FIELD CROPS FOR THE COTTON-BELT 



of the plant is such that all of the food materials must 

 be taken up in solution or, in the case of carbon and some 

 of the oxygen, as a gas. The mineral constituents obtained 

 from the soil are taken in by the root-hairs with the 

 stream of water. This dilute solution of food passes 

 through the soft outer tissues (cortex) of the root to the 

 vascular system through which it passes directly to the 

 leaves. 



In taking up food, roots exhibit a selective power in 

 that they take up from the soil certain elements to the 

 total or partial exclusion of others. For instance, from a 

 solution of sodium nitrate plants take up the nitric acid 

 and leave the sodium. The continuous absorption of 

 food and water by the cotton plant will depend upon cer- 

 tain external conditions such as the moisture content of 

 the soil, the nature and amount of plant food materials in 

 the soil, the temperature of tjie soil, the activity of trans- 

 piration, and the intensity of light. 



26. *The taking up of carbon. Approximately 50 

 per cent of the weight of a water-free cotton plant is car- 

 bon. The plant secures its carbon from the carbon dioxide 

 of the air. It is estimated that carbon dioxide exists in 

 the air in the ratio of about 3 parts in 10,000 or 0.03 per 

 cent. 



As shown in paragraph 11, one of the functions of the 

 leaves is to take up from the air the carbon dioxide needed 

 to build plant tissue. This process is greatly facilitated 

 by the large number of stomata that are thickly scattered 

 over the under-surface, and to a less extent, the upper- 

 surface of the leaves. One of the primary functions of 

 the stomata is to serve the plant as breathing pores. The 

 air containing carbon dioxide passes through the stomata 

 into the air-spaces of the leaf. From here the carbon 



