N. M. doMHRR 365 



and of little use to the plant, and that the formation of these phosphates 

 in superphosphates diminishes the value of the material to the plant. 

 On the other hand, direct ex])erimenti leaves no doubt that phosphates 

 of iron and aluminium are very useful to the plant as sources of phos- 

 phorus. There seems to be a prejudice, which prevails in spite of experi- 

 mental evidence to the contrary, that sparing solubility must mean a low 

 availabilitv. The e.xperimental fact is that sparint^ly soluble phosphates 

 are often easilv available, and the inevitable conclusion is that solubility 

 is not the dominatinji' condition of availability. 



A Modification of the Present Hypothesi.s. 



The hypothesis that all material entering the plant from the soil first 

 becomes dissolved in the soil water and then diffuses into the root hair, 

 arose on the basis of two fundamental beliefs. First, the physiological 

 belief that only material in true solution can diffuse through the cell 

 protoplasm and second, the belief that the soil is a simple system of 

 particles moistened by a solution. Now tliese two fundamental concep- 

 tions have become modified. Becaxise of the modifications and also 

 because of the cbscrepancies already noted between the facts and the 

 hypothesis, it is desirable that the hypothesis should be reconsidered. 



The absorption of colloids hi/ the plant. As a first point in the recon- 

 sideration it may be noted that the teaching that only material in true 

 solution can diffuse into the plant cells is not unquestionably true. There 

 is some eviflence that less highly dispersed material can enter. Czapek- 

 definitely states that colloids can enter the plant cells. "Die lebende 

 Plasmahaut ist nicht nur fiir echte Losungen, sondern auch fiir kolloide 

 Losungen durchlassig." He refers to the passage of dyestuffs and of fat 

 emulsions into plant cells. Pfeffer^ records the diffusion of silicic acid 

 into the cell sa]) of plants. 



There is also some recent experimental evidence in favour of the 

 absorption of colloidal silica by plants. Jennings* describes a series of 

 experiments in which wheat seedlings were grown in water solutions to 

 which were added various absorbing substances. When colloidal silica 

 gel was used there was a marked increase in growth and in drv matter, 

 and it was clearly shown that the plant had absorbed silica. 



' See Marais, Soil Set. 1922, 13, No. .") ;uui bibliography. 

 " Biochemie der PJIanzeii, 1M3. 

 ' Tlw Ptiysioloyij uf Plants. 1900. 

 « Soil Set. 1919, 7. 



