no BOTANY OF THE LIVING PI ANT 



hool ni botanists who have held that the- living cells 

 thai lie along tl oi the transpiration stream, both in the 



A and in the item, play e more important part in raising water 

 than the uphokta l >h< on Theory would ascribe to them. 



Absorption oi Salts prom thi Soil. 



Water is not the only substance derived from the soil. The root- 

 hairs also absorb a variety of inorganic salts that are present in 



solution in the soil-water (\ , Analysis of plants reveals that 



in addition to carbon (derived from atmospheric carbon dioxide) 

 hydrogen and oxygen (derived from water), a great variety of elements 

 may be detected within the tissues. Those most commonly present 

 are nitrogen, potassium, sodium, calcium, magnesium, iron, phos- 

 phoric, sulphur, silicon, chlorine, manganese, aluminium, zinc and 



on. All these elements arc derived from the soil, and are absorbed 



the plant in the form of salts. 



Information concerning the absorption of dissolved substances by plant 

 cells m general has been given on pp. 38-39. The extent of absorption 

 rtuular salt or ion by a plant is affected by its concentration in the 

 soil and by the permeability of the root-hairs towards it: also by the rate 

 at which it p from the root-hairs into the other parts of the plant, and 



1>\ the opcr.it ion of factors not properly understood and referred to on p. 39. 

 The entrance of salts into the root-hairs is a process essentially independent 

 of the entry of water. It could continue in the absence of the latter process, 

 while conversely the mere presence of a particular salt in the soil-water 

 does not 1 inly mean that the salt will pass into the root-hairs along 



with any water that is being absorbed. It seems probable, however, that 

 the movement of water will hasten the absorption of salts to which the root- 

 hair is permeable, as well as the subsequent transport of the salts to the 

 DO of the root. There ta fairly general agreement that the salts are carried 

 ,1 1 ) tM ' rial organs of the plant in the transpiration stream, as already 



mentioned. The structure of the endodermis necessitates that all materials 

 diffus i n g through this layer should pass through the protoplasts, wall-diffusion 

 ng prevented by the corky strips on the radial walls (p. 83). Thus only 

 materials to which the protoplasts are permeable can penetrate into the 

 inner part of the root and undergo transport to the other parts of the plant. 



elements listed above only seven are usually regarded as 



plant development, namely, nitrogen, phosphorus, 



sulphur, p mi, calcium, magnesium and iron. This view is 



based on riments in which advantage is taken of the fact that 



plants will grow with their roots in aqueous solutions instead 



ot in soil Any d( ombination of salts ran be supplied in solution 



