Bauhaus-Universität Weimar

Titel:
The Cyclopaedia of Anatomy and Physiology, vol. 4: Pla [corr.: Ple] - Wri
Person:
Todd, Robert Bentley
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit29465/452/
442 
SECRETION 
organ, however complex its structure may be. 
The progress of comparative anatomy has 
shown that neither the form nor the in¬ 
ternal arrangement of the parts of a gland 
could have any essential connection with the 
nature of its product (see Gland) ; since 
even those glands (the liver and the kidney, 
for example) in which there is the greatest 
complexity of structure, make their first 
appearance at the lower end of the animal 
series, as in the early embryo of the very 
highest, in the simplest possible form. Still 
something was wanting to prove that the 
structural elements immediately concerned 
are in all instances the same; and there 
seemed no analogy whatever between the 
secreting membrane of the animal and the 
secreting cell of the plant. The doctrine 
was first propounded by Purkinje * and 
Schwannf, adopted and extended by Henle 
and fully confirmed by the researches of 
Goodsir $ and Bowman ||, that the true pro¬ 
cess of secretion — under whatever form it 
may present itself—is always performed by 
the intervention of cells; which, as part of 
their own regular vital actions, select and 
withdraw certain ingredients from the nu¬ 
tritive fluids, and afterwards set them free 
again, generally by the rupture or dissolution 
of the cell-wall, but sometimes perhaps by a 
simple act of transudation. For the proper 
comprehension of this doctrine in all its ge¬ 
nerality, it is necessary to give some attention 
to the history of cell-development, as mani¬ 
fested in the simplest forms of organic ex¬ 
istence ; those cryptogamie plants, namely, 
in which every cell is a distinct and inde¬ 
pendent individual. 
The earliest condition of such a cell is a 
minute molecule, which cannot be discerned 
except under a considerable, magnifying 
power, and in which even the highest ampli¬ 
fication fails to exhibit any distinction of 
parts. When placed under circumstances 
favourable to its development,— namely, 
when supplied with the materials of its nu¬ 
trition, and stimulated by the requisite de¬ 
gree of warmth,—this germ increases in size; 
and a distinction becomes apparent between its 
transparent exterior and its coloured interior. 
Thus we have the first indication between 
the cell-wall and the cell-cavity. As the en¬ 
largement proceeds, the distinction becomes 
more obvious ; the cell-wall is seen to be of 
extreme tenuity and perfectly transparent, 
and to be homogeneous in its texture, whilst 
the contents of the cavity are distinguishable 
in the Algæ by their colour, which is green in 
the Chlorococci, and bright red in the Hæma- 
tococci ; but in the simple fungi, such as the 
Torula cerevisii, or yeasf plant, they are 
colourless. The contents of the cell-cavity 
* Isis, 1838, No. 7. 
+ Froriep’s Notizen, Feb. 1838. 
I____ ± a 1 • Ï OOO „ 1 A/1 
1842, “ On the Structure and Uses of the Malpighian 
Bodies of the Kidney.” 
have no relation whatever to the material of 
the cell-wall. Of this we have a remarkable 
example in the cases just cited ; for whilst 
the red and green coloured products of the 
Algae are probably nearly related to each 
other and to the chlorophyll of higher plants, 
being simple ternary compounds of water and 
carbon, the cell-contents of the yeast-plant 
are closely allied to the protein compounds ; 
and yet the cell-walls in both instances are 
composed of the same material, cellulose. It 
is evident, then, that the inherent powers of 
the cell are not confined to the application of 
nutrient materials to the extension of its own 
walls, and the consequent enlargement of its 
cavity ; but that they are exercised also in 
selecting from (and it maybe in combining or 
modifying) the same materials, in order to fill 
this cavity with a certain product, which may 
be altogether different in its constitution and 
its properties from that of which its wall is 
composed. This latter process is as essential 
to our idea of a living cell, as is the growth 
of its wall ; and must never be left out of 
view when the history of cell-development is 
being considered. 
The nature of the compound thus stored 
up in the interior of a cell depends in part 
upon the original inherent endowments of the 
cell itself, derived from its germ ; and, in 
part, upon the character of the nutriment 
supplied to it. Thus we find that the simple 
Algæ will grow wherever they can obtain, 
from the air and moisture around, the elements 
of their cell-walls and of their cell-contents ; 
which elements they have themselves the 
power of combining into those peculiar com¬ 
pounds, of which analysis shows that they 
are composed. But out of the very same 
materials, and under circumstances to all 
appearance identical, the Chlorococcus manu¬ 
factures a green product, and the Haemato- 
coccus a red one. On the other hand the 
yeast-plant, like the fungi in general, will 
only grow where it meets with an azotised 
compound already formed ; and from this it 
elaborates the product which occupies its 
cell-cavity, its cell-wall being apparently 
formed by the same process as that of the 
simplest Algæ. It could no more vege¬ 
tate, as they do, upon cold damp surfaces, 
than they could develop themselves in a 
solution of fermentible matter secluded from 
the light. 
A similar variety of function is seen amongst 
the cells, whose aggregation makes up the 
structure of any one of the higher plants, and 
which are all the descendants of the single 
cell which constituted its original germ. Thus 
we have in the green cells of the leaves the 
representatives of the simple Chlorococci ; 
these, under the influence of solar light, com¬ 
bining the carbon which they derive from the 
atmosphere, or from the soil, with the water 
transmitted from the roots, and elaborating 
these elements into a variety of new products, 
amongst which chlorophyll and cellulose are 
still prominent ; but also operating upon the 
azote which they draw from the atmosphere
        

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