PLANT NUTRITION. 13 
been said, the protoplasm, which: is the essential agent 
in all these processes ; but, subject to a few exceptions, 
which need not now be specified, this protoplasm is 
always shut up within a cell-wall. Nor is it absolutely 
necessary that there should be more than one cell. Most 
plants with which the cultivator has to do consist of ag- 
gregations of such, but there are myriads of other plants 
which consist of but one cell; in such a case the cell is 
the plant, the plant is the cell. Now this is important, 
because it shows us that all the processes of life can be, 
and often are, carried on in one cell only, that is, by one 
fragment of protoplasm. Where the fabric becomes 
more complex, one cell is more or less dependent on 
another, but still there is always a measure of indepen- 
dence left to each individual cell. Were it not so, the 
scythe of the mower or the grazing of the sheep, by de- 
stroying a portion, would kill the entire plant. 
It follows that the life-history of a plant is, in essence, 
the life-history of protoplasm and of its covering, the 
-cell-wall; and hence it is that the microscopist or the 
chemist in the laboratory studying what goes on in isolat- 
ed cells, placed as far as possible under uniform condi- 
tions, is really adopting the best means of investigating 
what takes place in the entire plant; a circumstance 
which the “‘ practical man,” so called, compelled to work 
in the field under such very different, more complex and 
much less definite conditions, finds it difficult to realize, 
Conditions of Diffusion.—Diffusion, it will readily be 
understood from what has just been said, is not equal or 
alike in all cases ; it depends upon the extent to which 
the two liquids are diffusible, upon their different densi- 
ties, upon temperature, and a variety of other conditions. 
So, in the case of osmosis, we have not only the nature 
of the two fluids to consider, but their relation to the 
membrane that separates them. The membrane may be 
