Page images
PDF
EPUB

Mirbel were chiefly owing to the developement of a spiral thread in the inside of the tissue; he, however, did not consider it in that light.

But although the origin of the different forms of tissue may be shown to be identical, it is obviously important to distinguish them for practical purposes. I shall therefore proceed henceforward to speak of them as if they were distinct in their origin.

Sect. I. Of Cellular Tissue, or Purenchyma.

CELLULAR, UTRICULAR, or VESICULAR tissue, generally consists of little bladders or vesicles of various figures, adhering together in masses. It is transparent, and in most cases colourless : when it appears otherwise, its colour is caused by matter contained within it.

If a thin slice of the pith of elder, or of any other plant, be examined with a microscope, it will be found to have a sort of honeycomb appearance, as if there were a number of

[merged small][graphic][merged small][merged small]

hexagonal cavities, separated by partitions (fig. 3.). These little cavities are the inside of bladders of cellular tissue; and the partitions seem to be caused by the cohesion of their sides, for if we boil the pith for a short time, the bladders readily separate from each other. In pulpy fruits, or in those which have their cellular tissue in a loose dry state when ripe, the bladders may be readily separated from each other without boiling. It was formerly thought that cellular tissue might be compared to the air bubbles in a lather of soap and water ; while by some it has been supposed to be formed by the doublings and foldings of a membrane in various directions. On both these suppositions, the partitions between the cells would be simple, and not composed of two membranes in a state of cohesion ; but the facility with which, as has just been stated, the cellules may be separated, sufficiently disproves these opinions. But although the double nature of the partitions in cellular tissue may be often demonstrated, yet the cellules usually grow so firmly together, that their sides really form in their union but one membrane; and it will be hereafter seen that in many cases the partition between two cells is originally simple.

The bladders of cellular tissue are destitute of all perforation or visible pores, so that each is completely closed up from its neighbour, as far as we can see; although as they have the power of filtering fluids with rapidity, it is certain that they must abound in invisible pores, and that they are not impermeable, as if they were made of glass. An opinion different from this has been entertained by some observers, who have described and figured perforations of the membrane in various plants. Mirbel states that “the sides of the bladders are sometimes riddled full of holes (fig. 4.), the fig. 4. aperture of which does not exceed the do

of a millimetre (or of half a line); or are less
frequently pierced with transverse slits, which
are occasionally so numerous as to fig. 5.
transform the bladders into a real
articulated tissue, as in the pith of
the Nelumbium (fig. 5.).” This

statement is now well known to have been founded upon inaccurate observation; what the supposed pores really are has already been explained. (See p. 5.)

With reference to this subject, it may be also observed, that the bladders often contain air-bubbles, which appear to have no direct means of escape, and that the limits of colour are always very accurately defined in petals, as, for instance, in the stripes of tulips and carnations, which could not be the case if cellular tissue were perforated by such holes as have

[graphic]

been described; for in that case colours would necessarily run together.

Cellular tissue is generally transparent and colourless, or at most only slightly tinged with green. The brilliant colours of vegetable matter, the white, blue, yellow, scarlet, and other hues of the corolla, and the green of the bark and leaves, is not owing to any difference in the colour of the cells, but to colouring matter of different kinds which they contain. In the stem of the Garden Balsam (Impatiens Balsamina), a single cell is frequently red in the midst of others that are colourless. Examine the red bladder, and you will find it filled with a colouring matter of which the rest are destitute. The bright satiny appearance of many richly coloured flowers depends upon the colourless quality of the tissue. Thus, in Thysanotus fascicularis, the flowers of which are of a deep brilliant violet, with a remarkable satiny lustre, that appearance will be found to arise from each particular cell containing a single drop of coloured fluid, which gleams through the white shining membrane of the tissue, and produces the flickering lustre that is perceived. The cause of colour in plants will be spoken of hereafter in the second book.

The manner in which cellular tissue is generated and grows, would appear to differ in different plants. Amici says that the new tubes of Chara appear like young buds, from the points or axils of pre-existing tubes, an observation which has been confirmed by Slack. It has been stated by Mirbel that the same thing occurs in the case of Marchantia polymorpha. That learned botanist, in the course of his inquiries into the structure of this plant, found that in all cases one tube or utricle generated another externally, so that sometimes the membranes of newly-formed tissue had the appearance of knotted or branclied cords. He satisfied himself that new parts are formed by the generative power of the first utricle, which spontaneously engenders on its surface others endowed with the same property. The amylaceous vesicles of malt in a state of fermentation manifestly produce new vesicles from their sides externally; and Turpin asserts that they also contain molecules, which are the rudiments of other cells.

This subject has lately engaged the attention of Professor Mohl, whose researches show that this mode of generating cellular tissue is far from universal, as might indeed have already been suspected from what is known of the formation of pollen grains. It appears that in Confervæ the increase in number of the cells takes place by the internal division of the parent cells. In Conferva glomerata the last joint is always as long as those below it, only rather more slender. The branches grow at the upper lateral extremity of a joint or cell; each at first is a small protuberance, which is transformed into a lateral cylindrical excrescence containing chlorophyll (green colouring matter), and having its cavity in communication with that of the joint which bears it; as the branch lengthens, a contraction is observable at the line of insertion, which contraction is directed towards the interior of the cell, and chokes up the green matter, forming a sort of partition, pierced in the middle like a ring. This partition grows with the growth of the branch, and at last completely cuts off all communication between the first cell and its branch. Thus cut off, the latter lengthens by degrees, till it forms a very long cylindrical cell, which divides in just the same manner into two other cells, the terminal of which alone lengthens, to be again bisected in its turn.

Schleiden's ideas as to the origin of cells are still different from all these: see page 20.

The bladders develope, in some cases, with great rapidity. I have seen Lupinus polyphyllus grow in length at the rate of an inch and a half a day. The leaf of Urania speciosa has been found by Mulder to lengthen at the rate of from one and a half to three and a half lines per hour, and even as much as from four to five inches per day. But the most remarkable instances of this sort are to be found in the mushroom tribe, which in all cases develope with surprising rapidity. It is stated by Junghuns, that he has known the Bovista giganteum, in damp warm weather, grow in a single night from the size of a mere point to that of a huge gourd. We are not further informed of the dimensions of this specimen; but supposing its cellules to be not less than the ado of an inch in diameter, and it is probable they are nearer the VO it may be estimated to have consisted, when full grown, of about

47,000,000,000 cellules; so that, supposing it to have gained its size in the course of twelve hours, its bladders must have developed at the rate of near 4,000,000,000 per hour, or of more than sixty-six millions in a minute.

Cellular tissue grows for a long time after its generation, and hence the bulk of a given part may be much increased without the addition of any new elementary organs. Link ståtes that in the branch of a Pelargonium cucullatum about 1 line in diameter, he found the larger cells qī of a line broad, which, in an older branch of the same plant, 2 lines in diameter, the larger cells were lo of a line broad; hence it was evident that the growth of the branch depended upon the growth of the individual cells.

The bladders of cellular tissue are always very small, but are exceedingly variable in size. The largest are generally found in the gourd tribe (Cucurbitaceæ), or in pith, or in aquatic plants; and of these some are as much as the zo of an inch in diameter; the ordinary size is about the zoo or the zdo, and they are sometimes not more than the Touo: Kieser has computed that in the garden pink more than 5100 are contained in half a cubic line.

Cellular tissue is found in two essentially different states, the membranous and the fibrous.

MEMBRANOUS CELLULAR Tissue is that in which the sides consist of membrane only, without any trace of fibre; it is the most common, and was, till lately, supposed to be the only kind that exists. This sort of tissue is to be considered the basis of vegetable structure, and the only form indispensable to a plant. Many plants consist of nothing else; and in no case is it ever absent. It constitutes the whole of Mosses, Algæ and Lichens; it forms all the pulpy parts, the parenchyma of leaves, the pith, medullary rays, and principal part of the bark in the stem of Exogens, the soft substance of the stem of Endogens, the delicate membranes of flowers and their appendages, and both the hard and soft parts of fruits and seeds.

It appears that the spheroid is the figure which should be considered normal or typical in this kind of tissue; for that is the form in which bladders are always found when they are generated separately, without exercising any pressure upon

U. B. GENT

« PreviousContinue »