252 THE MOVEMENTS OF WATER 







PART III. 



THE EXCRETION OF WATER IN THE FORM OF FLUID. 



SECTION 41. General View. 



The power of excreting water in the form of fluid is widely distributed 

 throughout the vegetable kingdom, and it may either take place from the 

 intact plant or may occur only after injury, as in the bleeding of cut stems. 

 In the former case the water escapes through stomata or other openings, 

 but both phenomena are closely related to one another, for it is owing to 

 the activity of living cells that water is forced into the vessels, intercellular 

 spaces, &c., from which it escapes in the fluid condition at the point where 

 least resistance is offered to its exit. It may happen that the water- 

 secreting cells lie on the surface of the plant and are directly exposed, in 

 which case special conducting channels and excretory pores are no longer 

 necessary and may be absent. Frequently again in leaves and flowers 

 special groups of epidermal cells, or single epidermal hairs, may have the 

 power of excreting watery solutions. The same power is also possessed 

 by the hyphae of various unicellular and multicellular fungi. 



The process is in all cases due to active exudation from living cells, 

 just as is the case in the glands of animals, and it is not always pro- 

 duced in the same manner. The excretion of water, except in nectaries, is 

 probably for the most part not dependent upon the presence of dissolved 

 substances outside the cells, but is due to internal causes originated by the 

 vital activity of the living protoplasts, the process being one of active excretion, 

 active exudation, or filtration under pressure. If the escape of water is however 

 due to the presence of osmotic substances outside the cell, then we may 

 speak of an osmotic withdrawal of water or of plasmolytic excretion. The 

 latter occurs in nectaries, for there the sugar present outside the cells acts 

 in the same way as a piece of sugar placed upon a freshly cut surface 

 of a beet-root or potato. The nectaries continue therefore to excrete 

 water even when the plant is flaccid, a fact of considerable biological 

 importance, whereas bleeding and the excretion from water-pores take 

 place only when the plant is fully turgid, for unless this is the case no 

 active exudation is possible. 



The actual excretion of water does not give any indication of the 

 causes which induce it. It is, however, certain that a dissolved substance 

 exerts the same equivalent osmotic action under all circumstances, so that 

 any one-sided accumulation of such substances will cause a flow of water 

 in a definite direction, provided that the cellular and other arrangements 

 interpose no insurmountable obstacle. No doubt the excretion may be 



