THE REQUIREMENTS OF PLANTS 



43 



In the bad years the average rainfall was 32*55 inches (harvest 

 years, September-August), while in the good years it was 27-10 inches ; 

 the badness of the season may be connected with the high rainfall and 

 corresponding low temperature. Similar results are obtained, however, 

 if other unfavourable conditions set in. 



The improvement in tone is well exemplified by the power of 

 resisting disease. At Rothamsted the potash-starved wheat is badly 

 attacked by rust, the mangold leaves by Uromyces beta, and the grass 

 by various other fungi, while the surrounding plots, equally liable to 

 infection, remain healthy. Growers of tomatoes under glass have 

 found that the ravages of various fungi and of eelworms are much 

 diminished by manuring the plants with potassium salts. 



Next to the sugar-producing plants, the leguminosae seem to stand 

 most in need of potassium salts. The potash-starved grass plots at 

 Rothamsted contain notably less clover than those fully manured, the 

 actual depression fluctuating according to the season. Some of the 

 weeds, especially the sorrel, require a good supply of potash. 



In absence of potassium salts mitotic cell division does not go to 

 completion ; Reed observed that the cell and nucleus both elongate, 

 but actual division does not occur (236). 



It is not at present possible to say whether all these phenomena 

 are different manifestations of one and the same specific action of 

 potassium in the plant, or whether there are several different causes 

 at work. 



Sodium can partially, but not completely, replace potassium as a 

 plant nutrient ; it thus delays the setting in of potash starvation, but 

 will not keep it off altogether. Hellriegel (131) found that sodium 

 salts always gave increases in crop even when potassium salts were 

 present in quantity. 



TABLE XIX. EFFECT OF SODIUM SALTS WITH SMALL AND WITH LARGE AMOUNTS OF 

 POTASSIUM SALTS ON THE GROWTH OF BARLEY. HELLRIEGEL (131). 



Breazeale (52) has more recently obtained similar results in water 

 cultures. It is well ascertained in farming practice that sodium salts 



4* 



