150 THE INDIVIDUAL ORGANISM 



deficit is the suction force which measures the power of the cell to take 

 up more water (always assuming that no change in volume of the cell 

 takes place). A cell with a suction force of 0.1 M will take water from any 

 soil solution having a molar concentration less than 0.1 M, and from 

 any adjoining cell that has a suction force of less than 0.1 M. These facts 

 find application not only in explaining the entry of water into roots but 

 in accounting for its movements within the plant. Osmotic pressure is 

 probably the chief agency in the entry of water. The walls of the delicate 

 epidermal cells and root hairs cannot withstand much pressure; the cells 

 would doubtless be ruptured were not water continually withdrawn from 

 them by the deeper tissues. 



The mechanism for such withdrawal and for producing a continuous 

 flow across the cortex to the stele is found in the gradient of increasing 

 osmotic concentrations and amounts of suction force that exists from 

 epidermis to stele, together with the mechanical suction exerted by the 

 transpiration pull — a matter which will be discussed in a later chapter. 

 In some plants the cells of the innermost layer of the cortical parenchyma 

 have about four times the osmotic concentration of the epidermal cells. 

 This means that each layer of cells exerts suction force on the cells toward 

 the outside of the root, while at the same time it loses water to the next 

 inner layer of cells. Along with the transpiration pull, this causes the 

 soil water entering the epidermal cells to move across the cortex toward 

 the vascular tissues of the stele. Here the water is pulled into the xylem 

 and is carried along the roots and up the stem to the leaves. The force 

 that causes the water to enter the xylem tubes seems chiefly that of the 

 transpiration pull exerted along the water columns in the tubes, as later 

 described. The transpiration pull is in turn the product of the suction 

 force of the cells in the leaf tissues, created in those cells by loss of water 

 in transpiration. 



Intake of Solutes. Much of the intake of dissolved substances by roots 

 seems to be a matter of simple diffusion. When the concentration of a 

 solute is higher in the soil solution than in the epidermal cells, its mole- 

 cules or ions diffuse into the cell faster than they diffuse out, raising the 

 concentration in the cell. If they are also diffusing into the deeper tissues, 

 the net result will be a flow into the plant. 



Diffusion, however, does not account for all the facts of absorption of 

 solutes. There is abundant evidence that molecules and ions of many 

 substances continue to enter the epidermal cells even when the concentra- 

 tion within the cells is higher than that in the soil water. This may go on 

 until the concentration in the cell reaches 25, 40, or even (in some fresh- 

 water plants) 1,900 times that outside. This active absorption, as it may 

 be called to distinguish it from simple diffusion, takes place against the 

 diffusion gradient, and must require the expenditure of energy. Proof 



