58 NINTH REPORT. 



ing, the pores in the membrane, (2) water and (3) a substance sohible in 

 water. This memljrane must be less premiable to the suljstance in sohition 

 than to water; and it is further assumed that the membrane is not ])remiable 

 to the same extent with all substances in solution. Now, if these three 

 things be arranged properly, osmotic action will be developed and a current 

 of liciuid started. Such is only the merest outline of osmotic action. But 

 the forces involved in such action have to do to a large extent both with the 

 absorption into the plant, and with the transportation of substances (in so- 

 lution) froni place to place within the tissues. The force which produces 

 osmotic action is a physical attraction existing mutually between j^articles 

 of one substance and those of another, the substilnce being endowed by 

 nature with this power or force. The ordinary cellulose cell wall is of course 

 readily pervious to water and to solutions; and within the tissues small 

 visible openings in the walls cause water interchange, and transfer from cell 

 to cell to be carried on the more readily. The protoplasmic membrane within 

 the cell immediately adjoining the wall, and also that bounding the vacuole 

 are both concerned with the transfer and with the al)sorption of water and 

 of solutions. Both membranes have to do with regulating tlie current through 

 the cells. These membranes possess a peculiar regulative power not oidy 

 with regard to the quality.of liquid but also to the quantity. It seems there- 

 fore to have some power of selection varying with the individual, or with 

 the species. But this power is very limited in extent. This regulative power 

 is well exemplified in sea-weeds and the like, which, though immersed in 

 water containing a high proportion of common salt, contain actually a very 

 small amount of this substance, and a much larger amount of potassium, 

 though the latter exists only in very small quantities in sea water. 



The most important factor having to do with the movement of water in 

 the water system of plants, is, therefore, osmotic action, and in the lower 

 forms of simple structure, it Is practically the only factor. Water plants 

 and land plants require, in a measure, separate treatment with respect to the 

 water systems, and, in view of the fact that water plants are comparatively 

 insignificant in economic importance, as compared with land plants, the latter 

 will be dealt with almost entirely here. 



When, in the process of evolution, plants took on a land habit, they de- 

 veloped, in consequence, structures which were more or less adapted to 

 the changing conditions, and such structures were accpiired only during 

 many ages of time. 8oil and air being the medium in which land plants 

 exist, the Avater required in the life of the plants must be obtained from these 

 two media. IBut, as air would be likely to contain liquid water only at inter- 

 vals, and the soil more likely to contain it contimiously, the soil became the 

 natural source of supply, and the organs concerned have been developed 

 accordingly. 



The underground structures mainly then are concerned with the absorp- 

 tion of liquid material, consequently the current of soil water through the plant 

 will be from the underground parts towards the parts above ground. And 

 if this current is to be sustained there must be a loss of water at some part 

 of the plant. The air favors this loss because of its affinity for water. But 

 it should be pointed out here that this liquid given out into the air is prac- 

 tically pure water, while that taken by the underground parts contain sub- 

 stances in solution in small quantity. 



In order that this transfer from the underground parts (chiefly roots) to 

 the parts (chiefly leaves) above ground, may be made economically certain, 



