196 TEXTBOOK OF BOTANY 



stances, move them where they are needed, and throw off those that 

 are not needed. The active agent may be the moving substance itself, 

 and the energy that impels it may be external to the plant. Obviously 

 such movements should be described not by terms implying that the 

 plant "takes up," "takes in," and "gives off" the materials, but by terms 

 implying that the materials are moving into, or out of, the plant. The 

 latter terms focus one's attention upon the really active agent, while the 

 former imply that the passive medium (the plant) is the active agent. 

 By common agreement the term absorption is used merely to indicate 

 that a substance enters the plant, without reference to the manner of 

 its entrance or the energy involved. 



Movements of materials and the water medium within plants. In addi- 

 tion to recognizing the energy relations of moving substances, it is also 

 important to visualize the water within the plant, since this water is the 

 medium in which substances move. We have already seen that leaves 

 are composed of cells. In each living cell of the leaf a central vacuole 

 filled with water and dissolved substances is surrounded by a thin film 

 of protoplasm, which is in turn enclosed by a cell wall composed of 

 cellulose and pectic compounds and sometimes of other substances also. 

 Similarly, all other organs of a plant are composed of cells. Water not 

 only fills the vacuoles of the cells, but it also surrounds and is between 

 most of the minute particles of which the protoplasm and cell walls are 

 composed. Cell walls become much thinner when the water in them 

 disappears by evaporation, just as a wet cardboard becomes thinner as 

 it dries. 



By weight, about 80 per cent of a growing corn plant in midsummer 

 consists of water. We may therefore think of a com plant as a branched 

 column of water held in place by exceedingly thin films of protoplasm 

 and cell walls. Substances that move into and within the plant are mov- 

 ing mainly in solution in this water. Substances that move out of the 

 plant are moving out of this water. The water also moves. 



These movements may be inhibited or retarded by layers of cork and 

 by cell walls that are highly cutinized because the fat-like particles in 

 these walls may not be embedded in water. Movement of materials into 

 and out of cells may be impeded also by the film of protoplasm in each 

 cell. This fact may seem strange because of the large amount of water 

 in protoplasm, but it is probably due in part to a greater accumulation 

 of fat-like substances in the outer surface layer of the protoplasm. For 

 the present, however, we may accept and use the fact without account- 



