There is only this one kind of elementary organ, which ~ 
We | differing in its chemical compo- | 
: sition from the contents of the™ 
ao aS Structural and Physiological Botany. 
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» apple, slices of leaves which are not too delicate, such as ~ 
the cabbage, etc. Since therefore, at least at a certain’ 
_ period of their existence, all plants consist of one or more. 
cells, these are termed the elementary organs of planta e 
may vary greatly in its form and behaviour, but is le 
the same in its essential nature. 
Until recently the essential constituents of the cell were 
considered to bea more or less firm cell-wall (Fig. 2 a), 
cell, a ce’/-fluzd enclosed by it, and 
the nucleus, c, occurring 10 ‘the 
latter. The cell-fluid was again 
divided into two principal ele- 
ments, the watery cedl-sap, and 
the mucilaginous semi-fluid Arvo- 
toplasm, b, in the clear transparent 
Fic. 2.—A cell from the root of imbedded a larger or smaller 
the lizard-orchis, Orchis hir- 
cina ; a the cell-wall, consisting number of granules, generally 
of cellulose ; 6 the protoplasm of very small size. Some even 
contracted by alcohol; c the , 
nucleus with a nucleolus. (x went so far as to consider the 
ap cell-wall the most important 
element, and the term cell was applied even to the walls 
of perfectly dead cells which had lost the whole of their. 
- contents. But since cells were subsequently observed, in © 
' which, at least for a certain period of their existence, such a : 
firm wall, differing chemically from their contents, was 
wanting, and since also there are cells in which a nucleus 
hyaline substance of which are 
is never found, it became necessary to modify the idea of - 
a cell, and the only essential constituent is now held to — 
be the protoplasm. In the vegetable kingdom this proto- A 
plasm never remains permanently uninvested by a firm 
envelope, but is always sooner or later enclosed in an — 
elastic more or less solid membrane, the substance of which _ 
