THE WATEli IN THE PLANT. 167 



in its interior, tlie so-called vacuoles. Now an examination 

 of the cells of rapidly growing tissues shows that their j^ro- 

 toplasm is much more watery than that of living, but dor- 

 mant tissues — e.g. , those of seeds — and one of the first signs 

 ■of activity in the latter is the imbibition of water. 



Tliis avidity of protoplasm for water plays an important 

 part in the general economy of the plant. By it all the cells 

 which contain protoplasm are kept turgid, and by the ten- 

 sion thus created the soft parts of plants are made rigid. 

 It plays no small part also in keeping up the supply of 

 moisture in living tissues when wasted by evaporation. (See 

 paragraph 220 et seq. ) 



216.— Water in the Cell-walls. In the cell-walls, accord- 

 ing to Nageli's theory, the water forms thinner or thicker 

 layers surrounding the crystalline molecules of cellulose. (See 

 paragraph 37, p. 32.) The wall of the cell is thus not a 

 membrane which separates the water of one cell cavity from 

 that in the next, but rather a pervious stratum, composed of 

 solid particles which are not in contact, and between which 

 the water freely passes. In a living tissue the water is con- 

 tinuoiifi from cell to cell, and constantly tends to be in equi- 

 librium — i.e., the turgidity of the cells is approximately 

 ■equal throughout the tissue, and likewise the wateriness of 

 both cell-walls and cell-contents. 



In the simpler aquatic plants the water of the cells and 

 iheir walls is continuous with that in which they grow. 

 Likewise the water in the tissues of roots or other absorbing 

 ■organs of the higher aquatic plants is continuous with that 

 which surrounds them; and even in ordinary terrestrial plants 

 there is a perfect continuity of the water in the root tissues 

 "with the moisture of the soil. 



217.— Water in Intercellular Spaces. In some cases the 

 intercellular spaces and passages, and even the vessels of the 

 more succulent plants, are filled with water, thus increasing 

 its amount in the whole plant very considerably. More 

 commonly, however, these cavities are filled with air and 

 gases, the vessels having early lost the protoplasm which 

 they contained at first. It is probable, moreover, that the 



