286 THE MICROSCOPICAL NEWS. 
there is a progressive increase in the quantity of water or decrease 
in the density of the layers from without inwards, the hilum being 
always the darkest and most watery part. That this is a correct 
explanation of the appearance of the layers may be seen by ob- 
serving the grainsinamedium—alcohol, for instance—which abstracts 
the water, and entirely, or almost entirely, obliterates the appearance 
of zoning. When the grains are allowed to dry, the striation is also 
very indistinct, and the place of the hilum is then usually seen to 
be occupied by a cavity containing air. A stratum of air also 
sometimes occurs between two layers, and brings them into view 
when they would otherwise be invisible. In the dry grains, too, a 
number of cracks may be seen radiating from the hilum, and pro- 
duced by the greater shrinkage of the central parts of the grain in 
consequence of the greater loss of water from them than from the 
outer layers. 
Concerning the mode of growth of the grain, and the origin of 
the layers, there has been, and is, much dispute. One set of 
observers state that the growth takes place as in a crystal, by accre- 
tion or deposition of layers, alternately more or less hydrated, on 
the outside of the grain, so that the outermost layers are the 
youngest. But Sachs, following Naegeli, maintains that the grains 
grow by intussusception or deposition of molecules in the interior 
of the grain between already existing molecules, and weighty argu- 
ments are adduced in favour of this view. 
The excentric position of the hilum is thus explained. The 
starch is never deposited in entirely dead and empty cells, but 
usually in cells whose vital activity is reduced, and but a lining zone 
of protoplasm remains. It is in this layer that the growth of the 
grain commences, and as it increases in size it is pushed towards 
the centre of the cell and is no longer entirely surrounded by pro- 
toplasm, and the part so removed from contact with the protoplasm 
naturally grows more slowly than the side that is still imbedded. 
Sometimes the starch is deposited in hollow vesicles in the proto- 
plasm, then as growth takes place the grain extends into the 
cavity away from the protoplasm, and the same irregular growth as 
before results. 
The hilum commonly conforms to the shape of the grain. In 
elliptical grains it is elongated in direction of the longest axis of © 
the ellipse, and in lenticular grains the hilum is lenticular. 
Occasionally, two or more nuclei appear in one grain, and con- 
centric layers are deposited around each. This is often seen in 
the haricot bean. When it occurs the most rapid growth usually 
takes place in the line joining the two nuclei, and a rupture at 
length takes place, whereby the original single grain becomes 
divided into two, though they may still remain in contact with each 
other. Such compound grains occur in Sarsaparilla. Examined 
