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VEGETABLE PHYSIOLOGY. 
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By ARTHUR HENFREY, Esq., F.L.S., Lecturer on Botany at St. George's Hospital. 
CTRCULATIOH" OP FLUIDS. 
'E last considered (p. 153) merely the circumstances connected with the absorption of fluid 
HJuf nourishment by the superficial cells of plants — those lying immediately in contact with the 
external supply. In the lower tribes of cellular plants, consisting simply of strings of cells or 
expanded plates of cellular tissue, all the cells are directly in relation to their external supply, and in 
most cases the individual cells seem to absorb and assimilate their food independently, as in the 
Confervacece and other simple Alga?. But as vegetables become more complex in their structure, some 
cells or collections of cells becoming devoted to the execution of one physiological function, others to 
another, it is evident that an interchange of the cell-contents must be required. Thus in the Lichens 
even, the plants present layers of cells of different characters, some appearing to be more particularly 
destined to the function of elaborating the crude nourishment, and as these furnish the supply to the 
rest, distribution of a simple kind undoubtedly exists here. We have no positive ideas respecting the 
laws which regulate this distribution ; all we know is, that it does take place, as may be proved by 
causing chemical solutions to be absorbed by particular parts of the cellular expansion composing the 
plant, and demonstrating their penetration into the other parts by the action of re-agents upon them. 
Such a distribution may be proved to take place in the Mosses also, for when the roots of these are 
placed in a solution of prussiate of potash, the presence of this salt can subsequently be detected in the 
leaves, and even the capsules, by the action of the test upon them, namely, sulphate of iron, which 
colours the fluid contents of the cells there. 
The distribution of the fluids absorbed by the roots becomes a still more difficult question when we 
come to examine it in the higher plants. With regard to the Monocotyledons, our knowledge of the 
facts even, is at present very imperfect ; in certain cases of bulbous plants, which have been investi- 
gated by the action of chemical tests in the manner just described, the solutions absorbed by the roots 
appeared to rise chiefly in the cellular portion of the fibro-vascular bundles of the bulbs and flower- 
stalks, passing upwards to be generally distributed throughout all the organs. In regard to the 
Dicotyledons we are possessed of many more facts, and these indicate that there exists there a series of 
phenomena much more complicated in their nature, and regulated by some laws which are at present 
involved in the greatest obscurity. 
It has been demonstrated by experiment, that the fluids absorbed by the superficial cells of the 
rootlets of Dicotyledons do not pass upwards in the rind of the roots, but make their way into the 
woody structure, even in the very small subdivisions of the roots, and through this woody structure 
pass upwards into the stem and branches. This is proved by two facts. If we remove a ring of bark 
down to the wood, from a growing tree, the course of the sap upwards is not interrupted ; but if we 
carefully cut through the wood of a similar tree, taking care to injure the bark as little as possible, all 
that portion of the tree above the wound soon dries up. 
From the wood of the stem the sap passes into the leaves, and here becomes spread through the 
soft cellular tissue, as is shown by the great evaporation of watery vapour from this substance. Before 
the sap has entered the leaves it is useless as nutrient matter, for the growth of a plant is arrested 
when we strip it of its leaves ; the fluid rising from the roots up into the leaves is therefore called 
the crude sajJ. In the leaves this undergoes a chemical alteration, which renders it capable of 
being applied to the nutrition of the growing tissues, ant then it passes hack from the leaves and 
descends to the lower parts of the plant through the bark. This is proved by the fact that the 
removal of a ring of bark arrests the growth of the parts situated below it, while the growth 
of the portion above the wound is accelerated by the accumulation of assimilated sap in it, 
its woody layers become thicker, it produces more fruit, and this ripens sooner. In an uninjured 
tree a portion of the descending sap, unused for the developement of new tissues, often returns 
into the wood, as is shown by the formation of starch in the horizontal medullary rings in 
autumn. Thus a kind of circuit is completed, not indeed in particular vessels, as in animals, but in a 
particular course through the different parts of the plant. Some doubt has been thrown on the 
existence of this circulation by recent writers, but the facts seem to admit of no other interpretation. 
The objections to the terms, ascending and descending sap, founded on the conditions exhibited by 
oi horizontal and pendent branches, are of no real value, since the main point of the question is not the 
absolute direction of the course in reference to the position of the plant upon the earth's surface, hut A 
the fact of the crude sap passing from the root to the leaves through the wood, and the assimilated sap h 
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