172 Plant Physiology 



of the membrane to substances like alcohol. There are, 

 however, serious objections to this idea. Reed found, 

 among other pathological conditions attending an insuffi- 

 ciency of phosphorus, that starch was transformed into 

 unusual carbohydrate forms, and that cell-walls were 

 often thickened. 



93. The r61e of potassium. Potassium is an essential 

 element, and the experiments which have been carefully 

 and accurately carried out make it possible to say that in 

 general there may be no fairly complete substitution of 

 potassium by means of the related metals, lithium, sodium, 

 rubidium, and caesium, and generally very slight partial 

 substitution among higher plants. It is, however, true 

 that, when potassium in sufficient quantity is not available, 

 the addition of sodium is almost invariably attended by 

 increased growth. The relation to sodium is discussed 

 more at length later. 



Potassium in organic food formation. Many investiga- 

 tors agree in assigning to potassium a peculiarly impor- 

 tant function in the formation of carbohydrates and pro- 

 teins. Loew and Reed have devoted special attention to 

 this point. When potassium fails, starch is not formed, 

 and even if sugar is furnished, proteins are not normally 

 produced. Cells in a condition to divide are also consider- 

 ably influenced by lack of potassium. Such cells might 

 elongate to twice their normal length, supposedly by a 

 process of stretching, but there would be no evidences of 

 cell or nuclear division. Loew regards the potassium as 

 a strong condensing agent (and he shows that in certain 

 cases potassium is able to effect changes which sodium 

 will not). Since condensation processes are probably 



