THE GENERAL STRUCTURE OF PLANTS 13 
itself in harmony with its environment. Finally, it carries 
out the processes of reproduction. 
The primary needs of a plant are fairly simple. If we 
study the life and the behaviour of one of the free-swim 
ming organisms of which we have already spoken, we see 
that its first requirement is water. In this it lives; from 
this it draws its supplies of nutriment and into this it pours 
forth its excreta. The arrangement of the protoplasm in the 
cell in one of the higher plants points toa similar need. If 
we regard the arrangement whether in the young or the 
adult cell, we notice particularly the very close relation of 
the protoplasm to water. The young cell enclosed in its cell- 
membrane speedily shows a tendency to accumulate water 
in its interior, and gradually drops appear in its substance 
which lead ultimately to the formation ‘of a vacuole always 
full of liquid (figs. 15, 16). This store of water in the 
interior of a cell is of almost universal occurrence in the 
lowly as well as the highly organised , 
plant. The constitution of proto- 
plasm, so far as we know it, depends A I \ Y 
4g OO . 
upon this relation, for the appa- 
rently structureless substance is 
always saturated with it. It is only 
while in such a condition that a cell 
can live; with very rare exceptions, 
if a cell is once completely dried, 
even at a low temperature, its life is 
gone, and restoration of water fails 
to enable it to recover. Fic. 16.—ADULT VEGETABLE 
The constancy of the occurrence Crus. x 500. (After 
of the vacuole in the cells of the yj, mee pay, peated 
vegetable organism is itself an evi- 4% 4y,yciens, mith mu 
dence that such cells are completely 
dependent upon water for the maintenance of life. The 
cell-wall, though usually 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 
