ng into annular. 458.
Scalariform ducts of a Fern; part of a bundle, prismatic by pressure.
459. One torn into a band.]

_Spiral_ ducts or vessels (Fig. 453-455) have thin walls, strengthened
by a spiral fibre adherent within. This is as delicate and as strong as
spider-web: when uncoiled by pulling apart, it tears up and annihilates
the cell-wall. The uncoiled threads are seen by gently pulling apart
many leaves, such as those of Amaryllis, or the stalk of a Strawberry
leaflet.

[Illustration: Fig. 460. Milk Vessels of Dandelion, with cells of the
common cellular tissue. 461. Others from the same older and gorged with
milky juice. All highly magnified.]

_Laticiferous ducts_, _Vessels of the Latex_, or _Milk-vessels_ are
peculiar branching tubes which hold _latex_ or milky juice in certain
plants. It is very difficult to see them, and more so to make out their
nature. They are peculiar in branching and inosculating, so as to make a
net-work of tubes, running in among the cellular tissue; and they are
very small, except when gorged and old (Fig. 460, 461).

Sec. 2. CELL-CONTENTS.

414. The living contents of young and active cells are mainly protoplasm
with water or watery sap which this has imbibed. Old and effete cells
are often empty of solid matter, containing only water with whatever may
be dissolved in it, or air, according to the time and circumstances. All
the various products which plants in general elaborate, or which
particular plants specially elaborate, out of the common food which they
derive from the soil and the air, are contained in the cells, and in the
cells they are produced.

415. =Sap= is a general name for the principal liquid contents,--_Crude
sap_, for that which the plant takes in, _Elaborated sap_ for what it
has digested or assimilated. They must be undistinguishably mixed in the
cells.

416. Among the solid matters into which cells convert some of their
elaborated sap two are general and most important. These are
_Chlorophyll_ and _Starch_.

417. =Chlorophyll= (meaning _leaf-green_) is what gives the green color
to herbage. It consists of soft grains of rather complex nature, partly
wax-like, partly protoplasmic. These abound in the cells of all common
leaves and the green rind of plants, wherever exposed to the light. The
green color is seen through the transparent skin of the leaf and the
walls of the containing cells. Chlorophyll is essential to ordinary
assimilation in plant