INTRODUCTION ΤΟ BOTAN Y. BOOK I. ORGANOGRAPHY; OR, OF THE STRUCTURE OF PLANTS. CHAPTER I. OF THE ELEMENTARY ORGANS. Ir plants are considered with reference to their internal organisation, 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; and if the enquiry 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 processes, 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 Tissu organique of Mirbel; and the Parties élémentaires, or Parties similaires, of De Candolle. The chemical basis of the elementary organs has been found to be oxygen, hydrogen, and carbon, with occasionally a little nitrogen or azote, combined in various proportions: their organic basis is membrane and fibre. The latter only are here to be considered. B It is a common opinion that membrane only is the basis of the tissue of plants, and that fibre is itself a form of membrane. But as we find both developed in many of the most imperfectly organised plants, such as Scleroderma and other fungi; and as 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 is obviously something superadded, it will be better to consider both membrane and fibre as the organic bases of vegetable tissue, rather than the former only. The membrane varies in 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, and sometimes having a perceptible green colour, and a thickness which is considerable if compared with the diameter of the cavity it encloses. It generally tears readily, as if its component atoms do not cohere with greater force in one direction than another; but I have met with a remarkable instance to the contrary of this in Bromelia nudicaulis, in which the membrane of the cuticle breaks into little teeth of nearly equal width when torn. (Plate I. fig. 6.) It is in almost all cases destitute of visible pores; although as it is readily permeable by fluids, it must necessarily be furnished with invisible passages. An opinion to the contrary of this has been held by some botanists, who have described the existence of holes or pores in the membrane of tissue, and have even thought they saw a distinct rim to them; but this idea, which probably originated in imperfect observation with ill-constructed glasses, is now generally abandoned. The supposed pores, with their rim, have been ascertained to be nothing but grains of semi-transparent matter sticking to the membrane: this has been proved by Dutrochet, who found that boiling them in hot nitric acid rendered them opaque, and that dipping them in a solution of caustic potash restored their transparency, - a property incompatible with a perforation; and any one furnished with a good modern microscope may satisfy himself upon the point, without going through Dutrochet's process; by simple movement in water the grains may be often detached. It however occasionally happens that holes do exist in the membrane, of which mention will be made hereafter. Fibre may be compared to hair of inconceivable fineness, its diameter often not exceeding the T20 of an inch. It has frequently a greenish colour, but is more commonly transparent and colourless. It appears to be sometimes capable of extension with the same rapidity as the membrane among which it lies, and to which it usually adheres; but occasionally elongates less rapidly, when it is broken into minute portions, and carried along by the growing membrane. In direction it is variable (Plates I. and II.); sometimes it is straight, and attains a considerable length, as in some fungi; sometimes it is short and straight, but hooked at the apex, as in the lining of the anther of Campanula; occasionally it is straight, and adheres to the side of membrane, as in the same part in Digitalis purpurea; but its most common direction is spiral. Whether it is solid or hollow has not been fully demonstrated; Dr. Purkinje asserts that it is hollow, as will be hereafter mentioned; but there can be no doubt that it is also, at least sometimes, solid, as in the fibrous cellules of the leaf of Oncidium altissimum; it is the opinion of many that it is hollow in the case of spiral vessels. Fibre has a constant tendency to anastomosing, in consequence of which reticulated appearances are occasionally found in tissue. The forms under which the elementary organs are seen are, 1. Cellular tissue; 2. Woody fibre; and, 3. Vascular tissue. SECT. I. Of Cellular Tissue. CELLULAR tissue (Contextus cellulosus or Tela cellulosa, Lat. ; Pulpa, Parenchyma, or pithy part, of old writers; Zellengewebe, Germ. ;) generally consists of little bladders or vesicles of various figures, adhering together in masses. Occasionally it is composed of fibre only, unconnected by membrane. It is transparent, and in all cases colourless: when it appears otherwise, its colour is always caused by matter contained within it. If a thin slice of the pith of elder, or 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 hexagonal cavities, separated by partitions (fig. 1.). These little cavities are the inside of cellules of cellular tissue; and the partitions are caused by the cohesion of their sides, as may be easily proved by boiling the pith a short time, when the cellules readily separate from each other. In pulpy fruits, or in those which have their cellular tissue in a loose, dry state when ripe, the cellules 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 been just stated, the cellules may be separated, sufficiently disproves these opinions. It is probable, however, that although the double nature of the partitions in cellular tissue may be demonstrated, yet that the cellules usually grow so firmly together, that their sides really form in their union but one membrane. Cellules 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 and is still entertained by some observers, who have described and figured perforations of the membrane in various plants. |