s: by its means, under the influence of sunlight,
the plant converts crude sap into vegetable matter.

418. Far the largest part of all vegetable matter produced is that which
goes to build up the plant's fabric or cellular structure, either
directly or indirectly. There is no one good name for this most
important product of vegetation. In its final state of cell-walls, the
permanent fabric of herb and shrub and tree, it is called _Cellulose_
(408): in its most soluble form it is _Sugar_ of one or another kind; in
a less soluble form it is _Dextrine_, a kind of liquefied starch: in the
form of solid grains stored up in the cells it is _Starch_. By a series
of slight chemical changes (mainly a variation in the water entering
into the composition), one of these forms is converted into another.

419. =Starch= (_Farina_ or _Fecula_) is the form in which this common
plant material is, as it were, laid by for future use. It consists of
solid grains, somewhat different in form in different plants, in size
varying from 1/300 to 1/4000 of an inch, partly translucent when wet,
and of a pearly lustre. From the concentric lines, which commonly appear
under the microscope, the grains seem to be made up of layer over layer.
When loose they are commonly oval, as in potato-starch (Fig. 462): when
much compacted the grains may become angular (Fig. 463).

[Illustration: Fig. 462. Some magnified starch-grains, in two cells of a
potato. 463. Some cells of the albumen or floury part of Indian Corn,
filled with starch-grains.]

420. The starch in a potato was produced in the foliage. In the soluble
form of dextrine, or that of sugar, it was conveyed through the cells of
the herbage and stalks to a subterranean shoot, and there stored up in
the tuber. When the potato sprouts, the starch in the vicinity of
developing buds or eyes is changed back again, first into mucilaginous
dextrine, then into sugar, dissolved in the sap, and in this form it is
made to flow to the growing parts, where it is laid down into cellulose
or cell-wall.

[Illustration: Fig. 464. Four cells from dried Onion-peel, each holding
a crystal of different shape, one of them twinned. 465. Some cells from
stalk of Rhubarb-plant, three containing chlorophyll; two (one torn
across) with rhaphides. 466. Rhaphides in a cell, from Arisaema, with
small cells surrounding. 467. Prismatic crystals from the bark of
Hickory. 468. Glomerate crystal in a cell, from Beet-root. 469. A f