STARCH-GRAINS. 
59 
grains would be found the outermost layer of which would be a watery one; this, 
however, never occurs; the outermost layer is always the densest and least watery. 
According to this supposition the nucleus would also possess the properties of the 
youngest grains, whereas the nucleus is always soft, the youngest grains dense. The 
theory of deposition could only explain the formation of partially compound grains 
if we suppose that the common layers had been subsequently deposited round two or 
more previously isolated grains ; but the com- 
mon layers would have a different form, and 
the fissures in the interior of such grains 
remain unexplained. The theory of deposi- 
tion, finally, is incompetent to explain why, 
in the secondary grains, the strongest growth 
always takes place in the line joining their 
^cToi (Fig. 49). The older hypothesis of a 
deposition of new layers from within pre- 
supposes that the starch-grains were at first 
hollow vesicles, which has never been ob- 
served; on this hypothesis, moreover, it cannot 
be explained how the phenomena arise which 
occur in the formation of secondary grains; 
and this hypothesis must moreover suppose 
growth by intussusception to explain the 
superficial extension of the layers. The hy- 
pothesis of growth by intussusception affords 
the simplest explanation of all the pheno- 
mena; and, after Nageli's researches, may be 
considered as a fully established fact. The 
material which penetrates into the grain, and 
there becomes deposited in the form of new 
particles of starch, is, of course, in solution ; 
but its chemical nature is not yet certainly 
known; dissolved starch can never be proved 
to exist in the plant, at least in those cells 
where active formation and growth of starch- 
grains has been observed. It is, however, 
probable that a solution of sugar contained in the protoplasm is the material out of 
which particles of starch are formed by chemical and physical changes. The starch 
is easily changed into sugar by different agencies. From various facts ^i^^'g- the pro- 
duction of radial fissures on drying), it must be concluded that the «^e ä ooufe s- of starch 
have not only a definite order of deposition in the direction of the radii, but are also 
arranged tangentially in a definite manner in each layer. A corresponding stratified 
structure with radial striation, and the consequent formation of areolae, has, however, 
been observed only occasionally and imperfectly. 
Growth by intussusception depends on the permeability of all parts of the grain to 
water and aqueous solutions. This can only be explained by supposing that the 
substance of starch is not continuous, but consists of distinct moloeutos , each of which 
possesses the .power of attracting water, and surrounds itself with an aqueous enve- 
lope ; the moToomos of starc^i are separated from one another by these aqueous 
Fk;. 49. — Starch-grams from the tuber of a potato 
X 800). A an older simple grain ; B a partially compound 
grain ; C, D perfectly compound grains ; E an older grain, 
the nucleus of which has divided ; a a very young grain, 
b an older grain ; c a still older grain with divided nucleus. 
envelopes ; the smaller the moTocuTos - in a given portion of a starch-grain, the thicker 
are these envelopes and the more watery the particular portion. From this it results, 
on purely mechanical principles, that, as the moToouf^ increase in size, the aqueous 
envelopes become thinner, and the mol^oulqn approach nearer one another. The 
watery layers therefore consist of small m^Tecules which are; separated by thick 
aqueous envelopes, the denser Ifess watery layers of larger moJoouIob with thinner 
