30 GROWTH OF THE PLANT 



extent this means that as the plant needs this or that 

 plant food the root hairs absorb it. For example, at any 

 one time nitrates may pass in freely and phosphates may 

 not. Later this proceeding may be reversed; or both may 

 enter. On the other hand this selective action does not seem 

 very perfect, for most soluble inorganic compounds will 

 enter a plant even when very harmful. Still further, the 

 protoplasmic layer may exert a sort of decomposing action 

 on a salt like sodium nitrate, absorbing the nitrate radicle 

 and leaving behind the sodium, combined probably as a 

 carbonate. Or potassium sulphate may be split up, the 

 potassium being absorbed in some other form, possibly as 

 the carbonate, and the sulphate left behind, probably as 

 sulphuric acid. Moreover, it prevents such cell contents as 

 sugar and soluble proteins from passing out. In general, 

 soluble organic compounds do not pass through the root 

 hair wall or protoplasm in either direction. In other words, 

 crop plants can not absorb organic compounds of the essential 

 elements, and organic compounds such as acids are not 

 excreted by the plant root hairs. The acid action of root 

 hairs is due to the excretion of carbon dioxide which in water 

 forms carbonic acid, capable of dissolving certain substances. 

 This excretion of carbon dioxide is the result of respiration 

 or oxidation of material within the root cells. 



18. Withdrawal of Water from the Roots. — Under abnormal 

 conditions, such as when the .soil moisture becomes too 

 concentrated in plant food, water passes out of the root 

 hairs instead of into them. This extends back from cell 

 to cell and results, if not remedied, in the ultimate death of 

 the plant. Too much fertilizer around the roots of plants; 

 strong brine solution, as when the ice-cream freezer is emptied 

 on the grass; white alkali in the west; all. these are familiar 

 causes of this effect of withdrawing water from a plant, 

 resulting in its death. This outflow of water, however, does 

 not prevent the dissolved salts from entering the plant at 

 the same time. In some instances the salts themselves, 

 passing into the plant in excessive quantities, stop the vital 

 activity of the cells. The so-called black alkali — or sodium 

 carbonate — of the west is one of these poisonous salts, unless 

 in very dilute solution. 



