14 VEGETABLE PHYSIOLOGY 



permeable, yet presents a certain obstacle to the absorption 

 of water, and so even those cells which are living in 

 streams or ponds usually possess a vacuole. Cells without 

 a membrane, such as the zoospores, already many times 

 mentioned, can more readily absorb water from without, 

 and hence they are not vacuolated to the same extent as 

 the former ones ; indeed, many of them have no vacuoles. 

 Where the vacuole exists it always contains water, so 

 that the protoplasm of the cell has ready access to it, as 

 much so indeed as the cell which possesses no wall. The 

 vacuole contains a store which is always available. 



The advantages which water supplies to the plants are 

 many. In the first place, we have seen there is a very close 

 connection between it and the protoplasm, the life of the 

 latter being dependent upon its presence. The information 

 we have at present does not enable us to explain the nature 

 of this dependence. There are other features of the rela- 

 tionship, howev er, into which we can enter more fully. The 

 protoplasm derives its food from substances in solution in 

 the water ; the various waste products which are incident 

 to its life are excreted into it and so removed from the 

 sphere of its activity. The raw materials from which cer- 

 tain cells construct the food which is ultimately assimilated 

 are absorbed from the exterior in solution in water. More- 

 over, water is the ultimate medium through which gaseous 

 constituents necessary for life reach the protoplasm. 



Passing from the consideration of the protoplasm in 

 particular, the plant as a whole shows a similar dependence 

 on water. Many parts owe their rigidity to the distension 

 of their cells by liquid ; growth of the different members is 

 dependent upon the same hydrostatic pressure. In many 

 cases communication between different parts of a plant is 

 brought about through the same instrumentality, and thus 

 the response of the plant to various forms of stimulation 

 is facilitated or indeed made possible. 



Another primal necessity of the plant is air. Every 

 living organism, with the exception of a few of the very 



