54 VEGETABLE PHYSIOLOGY 



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 mem- 

 brane, such as the zoospores already many times men- 

 tioned, can more readily absorb water from without, and 

 hence they are not vacuolated to the same extent as are 

 those which possess a cell-wall ; indeed many of them 

 have no vacuole. This cavity when present being always 

 filled with liquid, the protoplasm of the cell has ready 

 access to water, as much so indeed as the protoplast which 

 possesses no cell- wall. The vacuole contains a store which 

 is always available. 



The quantity of water which a vacuole can contain is 

 very small, and as the needs of the protoplasm are some- 

 what extensive, a need arises for the continual renewing of 

 its supply. This is evident when we consider that the 

 protoplasm draws its nutriment eventually from the water, 

 and that it must return to it such waste products as it 

 gives off. Its oxygen must be drawn from the same 

 source, for this gas can only pass into the interior of a cell 

 by entering into solution in the liquid which it contains. 

 In cells which are deep-seated the need of oxygen can only 

 be supplied by a slow passage from cell to cell of the gas 

 which has been dissolved by those abutting upon a free 

 surface. Similar considerations apply to the elimination 

 of the carbon dioxide which accompanies the respiratory 

 processes. 



The life of a plant is consequently very intimately con- 

 nected with the renewal of the water which the cells contain. 

 Fresh liquid must be taken in, and that which is already 

 there must be to a certain extent removed ; the plant 

 demands in fact a kind of circulation of water, and this 

 becomes the more imperative as the mass of the plant 

 increases, with the possible exception, however, of those 

 massive plants whose habitat is marine. 



In examining the way in which this circulation is set 

 up and maintained, it is first necessary to inquire into the 



