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If plants are considered with reference to their internal organization, they appear at first sight to consist of a vast multitude of exceedingly minute cavities, separated by a membranous substance; more exactly examined, it is found that these cavities have a variety of different figures, and that each is closed up from those that surround it; if the inquiry is carried still farther, it will be discovered that the partitions between the cavities are all double, and that by maceration in water, or by other methods, the cavities with their enclosing membrane may be separated from each other into distinct bodies. These bodies constitute what is called Vegetable Tissue, or Elementary Organs: they are the Similary parts of Grew.

The chemical basis of the elementary organs has been found to be oxygen, hydrogen, and carbon, with occasionally nitrogen or azote, combined in various proportions: their organic basis is solidified mucus, either in an independent state, or organized in the form of membrane and fibre.

Organic mucus has only lately been recognised as the pri

mitive condition of vegetable tissue; although it has long been known as a substance existing in Algaceous plants, prior to the appearance of organization, as in Protococcus nivalis, &c. It has been found by Brongniart, Henslow, &c. in the form of a thin homogeneous membrane, applied to the cuticle of the leaves of some plants, and only separable after maceration; it is probable that it constitutes the whole exterior surface of all plants, and that it is even drawn over the sacs which constitute hairs; I have found it distinctly on the petals of Hydrotænia meleagris (see Bot. Reg. 1838. misc. No. 128.), but its extreme tenuity and firm adhesion to the tissue below it renders it difficult to detect it; and there is no doubt that it occurs very generally in the interior of plants between their cells, filling up the intercellular spaces, and gluing together all the parts. Mohl, with his usual skill, has shown that this substance is found so frequently, that we cannot refuse to acknowledge its presence as a constant fact. The Box, and the young annual shoots of Sambucus nigra, are especially noticed as well suited to show this structure; it will be seen to form a considerable part of the mass of the albumen of Alstromeria salsilla, see fig. 2. c. where it is 3.500 of an inch in diameter. Valentin has measured the thickness of the intercellular organic mucus in several instances, and gives the following table of the proportion between it and the cells of certain plants, calculated in Paris inches.

Camellia Japonica
Hoya carnosa .
Magnolia grandiflora
Cestrum laurifolium
Daphne Laureola -
Pinus Picea -
Aloe intermedia -
Aloe lingua -
Agave Americana -

Thickness of the Size of Proportion of the intercellular mucus. the cell. Ist to the 2d. - 0,000112 0,000675 1 : 6,02 . 0,0001500,000650 1 : 4,33 - 0,000150 0,000425 1: 2,83 - 0,000275 0 ,001550 1: 5,63 - 0,000390 0,0010001 : 2,56 - 0,000450 0,001200 1 : 2,66 - 0,000775 0,002100 1 : 2,71 - 0,000825 0,002575 1: 3,12 - 0,000850 0,002375 1: 2,79

Meyen admits the fact of the presence of this intercellular mucus, but considers it a secretion from the sides of the cells. He particularly refers to its condition in the petiole of Beta cycla, in proof of the correctness of that view.

It is the opinion of some anatomists that of membrane and fibre, the latter only is the basis of the tissue of plants: fibre itself being a form of membrane. But we find both the one and the other developed in many of the most imperfectly organized plants, such as Scleroderma and other fungi, and it is difficult to conceive how that can be a mere modification of membrane which is generated independently of it, which has no external resemblance to it, and which in many cases is obviously something superadded.

Membrane varies in its degree of transparency, being occasionally so exceedingly thin as to be scarcely discoverable, except by the little particles that stick to it, or by its refraction of light, but in ferns, some fuci, and other cryptogamic plants, it is brown from its first birth: according to Röper it is green in Viscum album; Link says it is green in the leaves of Ruellia Sabiniana and the petiole of Cycas revoluta ; and Meyen mentions its being orange coloured in the petiole of many tropical Orchidaceæ. It is always excessively thin when first generated; and whatever thickness it afterwards acquires must be supposed to be owing to the incorporation or incrustation of secreted matter. This was first observed by Mohl in Palm-trees, where he found a successive addition of strata to the lining of the cavities of the cells; and is apparently an universal occurrence where membrane becomes thickened. But the matter added to membrane is often so homogeneous as to offer no trace of its being deposited concentrically, even when examined by the most powerful microscopes, and I am by no means able to discover the regular lines upon its section which are represented so uniformly by the German anatomists. There can, however, be no doubt that the membrane of the woody tubes of the fig. 1.

liber is in many plants thickened successively by the deposit inside of concentric layers of sedimentary matter, as may be seen in Castanea vesca (fig. 1.a), and Betula alba, and in the

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