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154 
VEGETABLE PHYSIOLOGY. 
cation between the absorbing roots and the leaves in which the assimilative and respiratory functions 
are principally performed. This is very evident when we remember the conditions in such plants as 
the Houseleeks, the Aloes, &c, where the stem can hardly be said to have a distinct existence, the 
leaves being closely packed upon a small conical body immediately in connection with the roots. 
Plants could not exist with such a reduction of the root-system ; when many leaves are formed there 
must be abundance of roots to supply them with nutriment. It is true, on the other hand, that the 
stem may in some cases be of more physiological importance ; but this is only when the leaves are so 
little developed that the stem is obliged to assume their functions, as is the case in the Cactus tribe. 
But these must be regarded as exceptions to the general rule of the subordinate physiological value of 
the stem, and as examples of the slight dependance of function upon form which exists in vegetables, 
compared with that which is met with in the organs of animals. 
The root while young is a soft cellular structure, clothed by an epidermis of great delicacy, in 
which no orifices exist ; as it grows older a solid case of wood is formed and increasingly developed in 
the interior ; and in plants that live for a succession of years, the outer portions of this woody struc- 
ture serve as the channels for conveying up the fluids absorbed by the young roots, which, as they are 
continually becoming elongated by the development of new cellular substance at the points, are 
removed further and further from the base of the stem which they supply. Now, as the epidermis of 
the roots presents no orifices, it is clear that all the food of plants must be taken up in a liquid or 
gaseous condition. The first question, therefore, in the study of the nutrition of vegetables is, what 
are the laws regulating the absorption of fluids by plants ? 
In the first place, what is the cause of the absorption of fluids by the cellular substance of the 
roots ? Is it dependent upon some peculiar vital attraction, or is it simply a result of the operation of 
the physical phenomenon of endosmose? The latter opinion is entertained by the majority of 
botanists at the present day, and indeed the cells of the roots do present all the conditions favourable 
to such a conclusion. They are filled with a fluid containing mucilaginous matter, rendering them 
more dense than the surrounding water containing merely a minute quantity of earthy salts in 
solution ; so that there is an attraction exerted upon the latter, drawing it into the interior of the 
cells. The facts, first mentioned by Saussure, that healthy and diseased or dead membranes possess 
very different powers of absorption, and that diseased membranes absorb water or weak solutions 
more readily than those which are perfectly healthy, tend to prove that the absorption does not 
depend upon the exertion of an active vital power, since we could hardly expect this to be increased 
by injury to the plants ; and on the other hand, the interruption of the healthy conditions may cause 
a relaxation of the natural tension of the membrane, or a disturbance of the position of the muci- 
laginous contents of the cells, which may favour the physical process of endosmose. 
An important question in reference to the supposed existence of a vital force of absorption is, 
whether plants are capable of exercising a selection in the fluids which they absorb. It is well 
known that different plants grown upon the same soil will often present very different mineral com- 
ponents in their ashes, and this can only be explained in two ways : either they absorb substances 
unequally, or they absorb all that is presented to them like a sponge, and excrete again all that is 
injurious or unnecessary to them. Saussure made experiments on this point without finding any 
distinct evidence of a selecting power ; but Trinchinetti, by growing different plants in mixtures of 
chemical solutions, obtained results which appear to indicate the existence of something of the kind. 
Thus, from a mixture of saltpetre and common salt, Mercurialis annua and Clienopodium virido took 
up much saltpetre and little salt, while Satureia hortensis and Solatium Lycopersicum took up little 
saltpetre and much salt. Again, from a mixture of sal-ammonia and salt, Mercurialis took up much 
sal-ammonia, and Vicia Faba much salt. Still we must not conclude from these experiments, that the 
plant has a selecting power, which makes it absorb useful and exclude injurious salts, since an experi- 
ment of Saussure's, in which he found that a poisonous fluid solution of sulphate of copper, was more 
readily absorbed than wholesome nutriment, would tend to show that the effect was dependent on 
some chemical and physical peculiarity of the substances, and their relations to the cell-membrane and 
its contents. It is certain that from some cause vegetable membrane is capable of excluding particular 
substances and abstracting the pure water from the solution, since Fungi have been observed to grow 
in solutions of arsenic ; and, in the experiments of Vogel, Cereus variabilis was watered for ten weeks 
with solution of sulphate of copper without absorbing any ; none was absorbed by the leaves of 
Stratiotes aloides ; and Cham vulgaris vegetated for three weeks in the solution of the same salt 
without taking up any. 
With regard to the theory supported by Liebig, that the plants excrete injurious or useless matter, 
the experiments of Macairc-Prinsep, from which it appeared that roots give off such substances, have 
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