^6 Introduction to Botany. 



zfi^ membrane, b, called the plasma membrane, a specialized 

 part of the cytoplasm ; the cytoplasm, c ; and the nucleus, d. 

 Plasma membrane, cytoplasm, and nucleus are alive, while 

 the cell sap, which occupies the remainder of the cell 

 cavity (all of the clear space within A\ and the cell wall 

 are not endowed with life. The plasma membrane is 

 readily permeable to water, but not to all substances 

 which the water may contain in solution. Its chief ser- 

 vice consists, not so much in keeping certain substances 

 from entering the plant, as in prohibiting the valuable 

 cell sap and portions of the living body of the cell from 

 passing out and becoming lost to the plant. Thus, while 

 vast amounts of water with substances in solution pass 

 into the plant through the root hairs, only very small 

 quantities of materials, useful in rendering soluble those 

 substances which the plant needs, are permitted to pass 

 out by the same channels. 



23. The Process of Absorption. — The process of the 

 passage into the root hairs of the substances dissolved in 

 the water of the soil is known as diffusion. The initial 

 force which causes this probably results from the energy 

 of motion of the molecules and ions of the diffusing 

 substance. Those molecules and ions which possess the 

 greatest energy of motion, or whose size and shape best 

 conform to the intermolecular spaces of the membrane, 

 will traverse the membrane most rapidly. When the 

 molecules and ions of a substance in solution are in equal 

 concentration, that is, are in equal number per unit of vol- 

 ume on both sides of the membrane, and their temperature 

 is the same, the number of them passing the membrane in 

 both directions per unit of time will be the same. This 

 is a state of equilibrium which can only occur between the 

 soil and the root hairs in the case of those substances which 



