238 D. R. Hoagland 



exudation from cut surfaces or in guttation, under appropriate 

 atmospheric conditions. The guttation water, and especially the 

 exuded sap, from cut stems, may contain certain solutes in much 

 higher concentrations than those of the culture solution. Upward 

 movement of mineral solutes can therefore occur under the in- 

 fluence of the forces of root pressure, closely related to the meta- 

 bolic activities of root cells. The results of some recent experi- 

 ments suggest that such solutes enter into the xylem system by 

 a polarized process involving metaboUc activities of root cells. 



Soil and Plant Interrelations 



Having surveyed in a general way the nature of the process of 

 absorption of mineral elements by plants, we must now give 

 some further consideration to certain interpretations in terms of 

 soil-plant relations, without, however, discussing the chemical 

 and physical system of the soil— a subject of extreme complexity. 

 The plant is generally compelled, by virtue of the properties 

 of the soil environment, to remove essential mineral elements 

 from dilute solutions. Some of the essential elements will ordi- 

 narily be present in the soil solution in extremely low concen- 

 trations. It is necessary for the rapidly growing plant to acquire 

 certain of these elements at rates which are extremely high in 

 comparison with those of the concentrations available. The plant 

 does in fact function as an active concentrating agent. (Its re- 

 markable efficiency in this respect has been referred to earlier.) 

 Active root growth also constandy brings into play new soil sur- 

 faces which yield solutes to the plant, and this exploration of the 

 soil is likewise dependent upon metabolic activities of the whole 

 plant. These considerations apply not only to the essential ele- 

 ments of older history, but also to other essential elements only 

 recendy recognized as essential, such as boron, copper, and zinc. 



