198 TEXTBOOK OF BOTANY 



ductive tissues of a plant from the roots to the leaves. This movement 

 is discussed in a later chapter. 



In Chapter VII we mentioned the fact that one of the results of oxi- 

 dation-reduction processes in living cells is a small electrical difference 

 between the parts of a cell and between the tip and base of a plant 

 organ. These electrical differences are mentioned here merely because 

 they may affect the mass movements mentioned above and also some of 

 the diffusion phenomena described below. However, these electrical 

 influences are not well enough known to be discussed in a general 

 textbook. 



Diffusion. All soluble materials that move into, out of, or within plants 

 move wholly or in part by diffusion. Plants may wither and become dry. 

 Water placed in an open dish slowly disappears. We cannot see the 

 water moving from the dish or from the plant, because it is moving in 

 the form of free molecules that have separated from the liquid mass. 

 It is diffusing from them into the air, and what we see is the result of 

 this diffusion. Later we shall see some of the results of the diffusion 

 of water into a plant, and from cell to cell within it. 



Instead of water we may place some aromatic liquid, such as pepper- 

 mint oil, in the open dish. In a short time the presence of this oil in 

 all parts of the room may be detected by its odor. Neither the oil nor 

 the water has been destroyed, but the molecules of each have become 

 widely dispersed in the air of the room. That is, they are now scattered 

 among the molecules of oxygen, nitrogen, and other gases in the air. 

 Similarly, the odor of flowers and fruits is the result of the diffusion of 

 some substance from them that we detect by our sense of smell. From 

 these examples it is perhaps clear that we are using the term diffusion 

 to refer to the dispersion of material by molecular movement. Although 

 we can neither see nor smell oxygen and carbon dioxide, there are ways 

 of proving that they also move into, within, and out of plants by 

 diffusion. 



If a colored salt ( potassium peiTnanganate, copper sulfate, etc. ) , that 

 is soluble in water is placed at the base of a column of water in a slender 

 glass tube, the result of the slow diffusion of the salt throughout the 

 entire column of water can be detected by its color. The molecules of the 

 salt move between the molecules of water. Even the force of gravity 

 does not prevent the upward movement of the molecules of a salt in 

 the water. Similarly, any substance that is soluble in the water of the 

 plant may move into the plant and from cell to cell within the plant by 



