498 
The Seed and its Germination 
other cases, as in the palms just mentioned, proteid matter is 
found not in grains, but in an amorphous form. 
These proteid reserve materials differ from all others in one 
very important respect. They contain nitrogen, which is an 
essential constituent of living substance. 
Some seeds — e.g. the pea — contain aleurone grains in the 
same cells as starch grains (fig. 15) ; others have the two re- 
stricted respectively to separate cells. 
Fat or oil is usually found saturating the protoplasmic or 
proteid network in the cells. Some seeds contain a very large 
proportion of this, the castor-oil plant, for instance, yielding 
50 to 60 per cent, of the dry weight of the seed. Hemp seed 
contains about 32 per cent. The pea, on the other hand, has 
not more than 2\ per cent. Generally, where oil is abundant 
starch is absent, though all three forms of reserve material may 
be present in the same seed. 
These substances — starch, proteid, fat — are all much alike 
in their insolubility in water. They resemble each other, too, 
in not being capable of what is called dialysis, a physical 
process on which much of the interchange of material in a 
plant depends. If a solution of sugar be placed in a vessel 
whose bottom is composed of parchment paper, care being taken 
to see that there is no pinhole or other aperture in it, and this 
be then floated on a vessel of water, it is found that soon there 
is a stream of water passing through the membrane in the 
direction of the sugar, and there is also a stream of sugar 
solution, though less in amount, passing through to the water. 
This goes on till the liquid on both sides of the membrane is 
of equal concentration. Sugar in this experiment is shown 
to be capable, when in solution, of passing through such a 
membrane. If, instead of sugar, a solution of proteid matter, 
such as white of egg, be placed on one side of the membrane 
while water is on the other, it is found that the egg albumin is 
unlike the sugar in behaviour and cannot pass through to the 
water. Sugar is dialysable ; albumin is not. 
This process of dialysis or diffusion is of great importance 
in the plant, as it is by virtue of it that the reserve matters are 
taken up from the endosperm by the embryo, and, indeed, that 
food material generally is caused to pass from cell to cell within 
the latter. If there be an accumulation in one cell of any 
material capable of dialysis, a stream will set in between it and 
the contiguous cell till both are equally saturated, and so 
the accumulated material will be gradually dispersed through- 
out the embryo, its ultimate course being guided by the con- 
sumption of it at some definite point. 
