VEGETABLE PHYSIOLOGY. 
267 
The proper contents of plants clo not form part of this fluid. They are contained in the cells, the 
little closed vesicles, and they are here frequently so completely retained, by some at present my sterious 
influence, that two of these little vesicles, whose membranes are freely penetrated by water, stand side 
by side, filled with most different matters, which never become intermixed. Each cell is, as it were, a 
little laboratory, in which it extracts from the constantly passing current of sap those constituents it 
requires for its own products; and when those products are completed, it either sends them on again 
in the fluid, or reserves them in store for a future need of some other part of the plant, or uses them to 
increase its own solidity, as in the formation of the thickening layers. It is probable, however, that 
the thickening layers in the older woody parts, are drawn from juices which have been elaborated in 
some other part of the structures. 
Young cells are found to contain at first only a colourless or yellowish viscid fluid, somewhat 
resembling white of egg in its character, being readily coagulated; this soon becomes granular, and 
after a time, as the cell grows, is found mostly as a layer spread over the inside of the cells. It is 
only so long as they retain this condition that they are capable of multiplying. If they are to be wood- 
cells, they soon lose this substance; but if they are to be “ working” cells, as we may term them, as 
in the leaves, and young roots, they retain it. In these we soon find starch grains; at first, small and 
few, then gradually increasing in number. After these come, in the green parts, chlorophyll granules, 
little round grains, or globules, of a green colour, and of a fatty nature. Although these substances 
form the most important matters of the cell-contents that can be detected by the microscope ; yet, in 
the general liquid, are found dissolved sugar and gummy matters, which are equally essentially 
concerned in the nutrition of plants. 
The real history of these products is yet to be made out; but we do already possess many 
contributions to it, and an approximative view of the process of nutrition may be expressed as 
follows :—■ 
We know that plants require water, carbonic acid, and ammonia, for their growth, besides certain 
earthy and alkaline substances, which may be left out of view for the present. Water is composed of 
oxygen and hydrogen; carbonic acid of carbon and oxygen ; ammonia of hydrogen and nitrogen. The 
viscid fluid or mucus contained in all active cells, is composed of oxygen, hydrogen, carbon, and 
nitrogen, and is, therefore, readily produced from the fluid containing all these elements absorbed by 
the roots. The membranes and woody thickening layers of the tissues, starch, sugar, and dextrine 
(the gummy matter), are composed of oxygen, hydrogen, and carbon, and are probably secreted or 
thrown off by the mucus, having passed through that stage first. While the plant is growing actively, 
the mucus matter is continually receiving new supplies of carbonic acid and water, from which the 
membranes, sugar, and dextrine, may be formed; while the ammonia, received in smaller quantity, 
permits the increase of mucus itself to furnish contents for new cells. But the supply being still 
greater than the demand for the products in the shape of cell membrane, or mucus, a large portion of 
the material for the first is stored up in the form of starch grains, and of the fatty matter, of which the 
chlorophyll globules chiefly consist; while the nitrogenous matters are deposited in the shape of the 
green colouring matter of chlorophyll, which is composed of all four elements : these processes being 
accompanied by the separation of oxygen, and going on under the influence of the sun’s rays. The 
mucus and the sugar and gum are carried freely onward in the flow of the general liquid upward. 
The starch and chlorophyll remain in a solid form in their cells until wanted, increasing in abundance 
according to the healthy vegetation of the plant. In autumn the starch begins to disappear from the 
leaves, being dissolved into gum or sugar, and carried into the buds, the tubers if present, into the 
roots or bark ; in fact, into any part which may be peculiarly the reservoir of nutrition for the new 
products in the following spring; of course, also into the seeds, where it is again deposited either in 
the shape of starch, or of fixed oils (which are analogous in composition), or of soft cellular substance, 
nearly filling up the cells, and capable of being re-dissolved, like starch in the spring. The leaves lose 
their green colour at the same time ; but this is at present a mysterious point, for the chlorophyll is not 
absolutely and wholly removed, the fatty substance, of a yellowish or reddish colour, remaining after 
the loss of the green colour by the dead leaves. 
Thus carbonic acid and water, mixed with certain earthy salts, are absorbed by the roots, the 
mucus is continually increasing these in the rapidly multiplying cells of the delicate tissue of the 
extremities of the roots; from these the sap pours onwards by endosmose throughout the whole struc¬ 
ture of the plant, until it reaches the evaporating surfaces of the leaves or green stem ; in its transit 
the wood-cells subtract from it the ternary compounds of oxygen, hydrogen, and carbon, from which 
are produced the new layers of cellular membrane, increasing the thickness of their walls. The green 
layer beneath the bark, and the tissue of the leaves, elaborate the same substances into starch and 
