THE MATERIAL INCOME OF PLANTS 



301 



among the particles and separates them more or less widely. But there 

 comes a limit to the swelling, and no more water enters. If it is removed, 

 the body regains its form and the particles, presumably, their identical 

 position. In solution there is no limit to separation, except by the 

 amount of water present; and when it is removed, the particles rearrange 

 themselves in forms which may be similar to those of the original body, 

 but are obviously not identical with them. Yet swelling may become 

 excessive, as when starch grains are put into hot water or alkalies, and 

 after certain limits are passed the swollen grain will not regain its normal 

 form. By such transitions imbibition merges almost insensibly into 

 solution. 



Relations of inner and outer water. — For further understanding it is 

 useful to attempt to picture the relations of the water to the other com- 

 ponents of a young cell immersed 

 in natural water. The outside 

 water has particles of many sorts 

 scattered through it ; for no 

 matter how pure, in nature all 

 water is really a dilute solution 

 of various substances. The water 

 of the cell wall has so many par- 

 ticles of cell-wall stuff scattered 

 through it that nearly half the 

 volume is cellulose; but it is con- 

 tinuous with the water outside. 

 The water of the cytoplasm and 

 of its inclusions is freer of these 

 substances, i.e. it is more nearly pure, because the cytoplasmic particles 

 form only about one fifth of the whole mass. This water, too, is con- 

 tinuous with the water of the cell wall, and with that of the solution 

 outside. The water of the vacuole is still less encumbered with other 

 particles, only one or two per cent, perhaps, but these are of diverse 

 kinds, for the cell sap is a solution of many things. The water here is 

 likewise continuous with that outside through the cytoplasm and wall 

 (fig. 620). 



Continuity of water. — The picture sketched above may be applied 

 to any plant cell by modifying it to fit special features, and may furnish 

 a working hypothesis, crude though it be, of the invisible structure of 

 organic bodies in general. This hypothesis is conceived to coordinate 



awe p t v 



Fig. 620. — Diagram of an imaginary sec- 

 tion through the cell wall and protoplast to 

 show the possible relations of water to the cell ; 

 a, outer water ; w, cell wall ; e, ectoplast ; 

 p, general cytoplasm ; t, tonoplast ; v, vacuole 

 (inner water); e, p, t, belong to the protoplast. 



