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 
a 
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. 
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 
