360 T/ir Arai/dhifili/ of Minerdl Plant FiukI 



The followini;; are some of Jennings' figures: 



Dry weights and silica contents of wheal seedlings grown in nutrient 

 solutions and in nutrient solutions containing silica (Jennings). 



Dry weight Silii-a in 

 of tops dry iiiatt<'r 



XT 1 - * 1 i- o- L -II- ( witliout silit-a 



Nutrient solution 8.) pts per million , ,. ■ , ,, 



' ' I + silica gel 1 "„ 



.,-,, < without silica 



( + silica gel 1 ",j 



-„„ (without silica 



.. ^>00 ,, -. .|. t I II 



I + silica gel I "„ 



|.N.,.. (without silica 



" " ] -f- silica gel 1 ",, 



Thus it appears that colloidal j^els can enter the plant, and entering 

 the plant from the soil they may carry with them absorbed substances. 



The essential elements for plant growth. It is commonly held that there 

 arc ton essential elements involved in the growth of plants. That tenet 

 implies tliat all other elements universally found in plants grown in soil 

 are there by accident. That this teaching has not been thoroughly 

 acceptable is well known and from a studv of plants grown in soil it has 

 been contended tliat sili(ton and a number of other elements are essential. 

 The argument for ten essential elements and no more is, of course, the 

 fact that apparently normal ])!ants can l)e grown in water solutions with- 

 out the introduction of any detectalile tia('e of elements other than the 

 well-known ten. That fact proves that only ten elements are required for 

 the growtli of plants in water solutian. The application of the same con- 

 clusion to plant growth in soil is entirely dependent upon the assumption 

 that the mechanism of feeding is the same in the soil as in water solution. 

 If the mechanism of feeding is different the chemical requirements may 

 well be different too. If, for instance, silica acts as a kind of carrier of 

 mineral substances to plants grown in soil, silicon will be an essential 

 element for that purpose. By considering the possibihty of a mode of 

 absorption from soil different from that which obtains in the case of 

 plants grown in water solution, some explanation of the additional ele- 

 ments always present in soil-grown plants may be found. 



The relation of the root hairs In the soil particles. Another point arises 

 from the modified view- of the constitution of the soil. Whereas the soil 

 system was formerly regarded as merely a system of moistened particles 

 it is now regarded as a system of particles which are coimected with the 

 external aiul free water by gel material. Some attention has been given 

 to the bearing of this conception of the soil on transpiration and the 



