72 SOIL CONDITIONS AND PLANT GRO WTH 



to the bacterial stripe disease : the numbers of plants affected 

 out of a total of 1 20 in each plot were : l 



Potassic fertilisers often afford the simplest method of dealing 

 with fungoid diseases and they are usually more effective than 

 other fertilisers under glass. 



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 accord- 

 ing to the season. Some of the weeds, especially the sorrel, 

 require a good supply of potash. 



There is some controversy as to whether potassium plays 

 any important part in protein synthesis in plants. 2 



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. Zwaardemaker 3 puts 

 forward the interesting suggestion that the potassium ion 

 (which is somewhat radioactive) may be replaced by any 

 other radioactive element, light or heavy, or by free radio- 

 active radiation, provided the doses are equi-radioactive. 



Sodium does not appear to be essential even to salt marsh 

 plants, although salicornia grew better in presence of salt 

 than in its absence. 4 It can partially, but not completely, 



1 S. G. Paine and W. F. Bewley, Annals of Applied Biology, 1919, 6, 185. 

 2 J. Stoklasa, Biochem. Zeitsch., 1917, 82, 310-323 ; T. Weevers, ibid., 1917, 



78, 354- 



3 y. Physiol., 1920, 53, 273. 



4 A. C. Halket, Annals of Botany , 1915, 29, 143-154. 



