56 
POPULAR SCIENCE REVIEW. 
but if you could compare it to some object that lias come 
under our observation, I think you might render it still more 
comprehensible to us all.” 
“Well,” I continued, after a little consideration, “I will 
endeavour to be more familiar in my exposition, and will give 
you a simile, uot a very pleasant one perhaps, but one that will 
enable you to form a clear conception of its structure. You 
have seen those little gelatinous capsules containing castor-oil, 
sold by druggists, have you not ? Suppose the gelatinous 
husk or shell of one of these to be thick, soft, transparent, and 
very elastic ; the contained castor-oil to be more viscid than it 
is, and floating in it a number of little green granules ; lastly, 
imagine the whole capsule to be globular, and so small as to 
be invisible to the naked eye ; — you will then be picturing to 
yourself that wonderful little mechanism, of which the secret 
spring is ‘ life/ the lowest living thing that has the power of 
growth and reproduction.” (PI. vi. fig. 2.) 
“ But you just now described one of these cells as e a closed 
imperforate sac/ how, then, can it grow and reproduce? 
indeed, how does it imbibe nourishment ?” 
“ I will answer the last question first. If you fill a bladder 
with water, are you not aware that the moisture will pass 
through it, although it is closed?” 
“ Certainly ; and that reminds me that I have often been 
surprised at the manner in which water exudes through those 
earthenware 1 water- coolers,’ giving to the material of which 
they are made a deeper tinge than when it is dry.” 
“ Precisely so,” I continued ; “ and this is called exosmosis, 
from two Greek words, that signify, ‘ an impulsion outwards.’ 
If, however, the bladder or earthenware bottle were empty, and 
you immersed it in water, the moisture would in like manner 
penetrate into the interior, and this is called endosmosis, or 
f impulsion inwards ; ’ but we may go still further. ‘ If two fluids 
of unequal densities are separated by an animal or vegetable 
membrane, the denser will attract the less dense through the 
membrane that divides them. This property is called endos- 
mose, when the attraction is from the outside to the inside ; and 
exosmose, when it operates from the inside to the outside of the 
body acted upon.’* This is the principle — now for its appli- 
cation. The vegetable cell, as I told you, contains a dense 
fluid, and so it attracts the moisture from without through its 
capsule or cell-wall, draws from it the materials necessary for 
its growth (ammonia, carbonic acid, &c.), and gives out oxygen 
through the same medium. This is the mode in which a cell 
is nourished ; and, although it would be too great a digression 
* Brande. 
