ON STARCH. 
257 
chlorophyll has produced starch under the influence of light 
are again placed in the dark, the starch is absorbed and 
disappears completely from the chlorophyll grains. From 
these facts it appears that the formation of starch is a 
function of chlorophyll exposed to light, and its disap¬ 
pearance a function of chlorophyll not exposed to light. 
Kraus found that in plants of Spirogyra which had lost 
starch after exposure to dark, the formation of that sub¬ 
stance in the chlorophyll grains recommenced in five minutes 
in direct sunlight, and in two hours in diffused daylight. 
Starch alwa} r s appears in an organised form as solid 
grains having a concentrically stratified structure, which 
arise at first as minute masses in the protoplasm, and con¬ 
tinue to grow while lying on it; their growth stops when 
they cease to be in contact with the protoplasm and when 
they have reached the cell-sap. Every grain of starch 
consists of a hydro-carbon, water, and a small quantity of 
mineral matter (ash). The hydro-carbon has the same per¬ 
centage composition as cellulose, to which it bears a 
strong resemblance. The starch, however, occurs in two 
forms in each grain : one easily soluble, which yields a fine 
blue with free iodine in solution (granulose), and the other, 
which in its composition resembles the cell wall substance 
(starch-cellulose). These occur together at every point of 
the grain ; if the granulose is extracted the cellulose remains 
as a skeleton, which shows the structure of the grain, its 
total weight being only two to six per cent of the whole. 
Since granulose preponderates, iodine solution colours the 
whole blue. Starch grains have always more or less rounded 
forms, and their internal organisation has reference to a 
centre of formation lying within themselves. The young 
grains appear to be always spherical, but since their growth 
is scarcely ever regular, their form changes to ovoid, ovate, 
rounded polyhedral, &c. Careful microscopical examination 
shows that each grain has water distributed throughout its 
organisation ; every point contains water as well as cellulose 
and granulose. Usually the amount of water increases from 
without inwardly, and attains a maximum at a fixed point. 
The cohesion and density decrease with the increase of 
water, as also the index of refraction, on which partly 
depends the power of perceiving these properties. The outer¬ 
most and least watery layer is succeeded by a sharply defined 
watery layer, and so on until the nucleus is reached, which 
is a very watery part surrounded by a less watery one. 
Although every layer is disposed around this centre, yet they 
are not continuously developed around the whole nucleus. 
