KEY TO STRUCTURAL, PHYSIOLOGICAL, AND SYSTEMATIC BOTANY. 1. PLANTS are not separable from animals by any absolute character; the simplest individuals of either kingdom not being distinguishable by our senses. 2. Animals are for the most part incapable of multiplying by mechanical or spontaneous division of their trunk, and are supported by nutritious matter, carried into their system from an internal bag or stomach. 3. Plants are for the most part congeries of individuals, multiplying by spontaneous or artificial division of their trunk or axis, and are supported by nutritious matter conveyed into their system by the absorption of their lower extremities or roots. 4. Generally speaking, the latter are fixed to some substance from which they grow, are destitute of locomotion, and are enabled to digest their food by the action of light upon their cuticle (38.). 5. Plants consist of a membranous transparent tissue, chemically composed of a hygrometrical combination of oxygen, hydrogen, and carbon, to which nitrogen is occasionally superadded. They are also found to contain many mineral substances, which they are supposed to separate from their proper food during the process of digestion, and to deposit in their tissue (216.). 6. Their tissue appears under three forms, viz. cellular tissue, woody tissue, and vascular tissue. These are called elementary organs. I. ELEMENTARY ORGANS. 7. Of these CELLULAR TISSUE (Tela cellulosa, Lat.; Tissu cellulaire, Fr.; Pulp and Parenchyma, of old writers; Zellgewebe, Germ.) is the only form universally found in plants; the other forms are often either partially or entirely wanting. 8. Cellular tissue is composed of transparent vesicles, the sides of which are not originally perforated by visible pores (17.). 9. Each vesicle is a distinct individual, cohering with the vesicle with which it is in contact. 10. Therefore, the apparently simple membrane that divides. two contiguous cells is in fact double. 11. If the adhesion of the contiguous cells be imperfect, spaces will exist between them. Such spaces are called intercellular passages. 12. The vesicles of cellular tissue, when separate, are round or oblong; when slightly and equally pressed together, they acquire a dodecahedral appearance, with a hexagonal section; stretched lengthwise, they become prismatical, cylindrical, or fusiform. 13. Cellular tissue, the vesicles of which fit together by their plane faces, is called parenchyma. 14. Cellular tissue, the vesicles of which are elongated and overlie each other at the extremities, is called prosenchyma. 15. Parenchyma constitutes all the pulpy parts of the medulla or pith (82.), the medullary rays (113.), a portion of the bark (102.), and all that is interposed between the veins of the leaves and of other appendages of the axis. Consequently it is found in every part of a plant, and especially in those which are succulent, such as the pulp of fruit and all the soft parts. It, however, sometimes acquires excessive hardness, as in the stones of fruits, and the bony skins of some seeds. 16. Prosenchyma is confined to the bark and wood, in which it is mixed with woody tissue (19.). 17. The function of the cellular tissue is to transmit fluids in all directions; the membrane of which it is composed is, therefore, permeable, although not furnished with visible pores (8.). 18. Cellular tissue is self-productive, one cell generating others upon its surface, either externally or internally. The first mode has been witnessed in Chara and Marchantia, the latter is a matter of inference. 18. a. Vasiform tissue is a modification of the cellular, consisting of short truncated cylinders placed end to end, and forming continuous tubes, in consequence of the ends of the cylinders being ruptured and opening into each other. It is common in wood, of which it forms what is vulgarly called the porosity. Its office is to convey fluids with rapidity in the direction of the woody tissue that surrounds it. Formerly it was considered a form of vascular tissue, and called dotted ducts. 19. WOODY TISSUE (Vasa fibrosa, Lat.; Tissu cellulaire allongé, Fr.; Clostres, Fr.; Baströhren, Germ.) consists of elongated tubes tapering to each end, and, like the vesicles of cellular tissue, imperforate to the eye. 20. It may be considered a form of the cellular tissue itself, to which it is frequently referred; but it is practically distinguished by its cylindrical form, great length, extreme fineness, and toughness ; the latter of which properties is produced by the thickness of its sides. 21. It is found in the wood, among the parenchyma of the liber (104.), and in the veins of the leaves, or other appendages of the axis. 22. Its functions are to give strength to the vegetable fabric, and to serve as a medium for the passage of fluid from the lower to the upper extremities. 23. VASCULAR TISSUE (Trachées, Fr.) consists of very thinsided cylinders tapering to each end, and having a spiral fibre generated in their inside. 23. a. Of this spiral vessels (Vasa spiralia, Lat.; Spiralgefässe, Germ.) are the type. Their fibre is of a highly elastic nature, and is capable of unrolling when stretched. 24. Spiral vessels are found in the medullary sheath (86.), and in all parts that emanate from it in an ascending direction, viz.: the veins of the leaves, and every thing that is a modification of them. 25. They are generally not found in any part which is formed in a downward direction; and are therefore usually absent from the wood, bark, and root. They, however, occur in these and other unusual parts in a few extremely rare cases; as in the wood, and bark, and pith of Nepenthes, and in the roots of many monocotyledons. 26. The spiral vessels convey air containing 7 or 8 per cent. more oxygen than the atmosphere. 27. They scarcely exist except in plants propagated by the agency of sexes. 28. Hence the two primary divisions of the vegetable kingdom, viz. Vasculares, or plants funished with spiral vessels, and propagated by the agency of sexes; and Cellulares, or plants destitute of spiral vessels, and not propagated by the agency of sexes. 29. Ducts (Fausses trachées, Fr.; Saftröhren, Germ.) are transparent tubes, the sides of which are marked with rings, bars, or transverse streaks. 30. They are slight modifications of the spiral vessel, differing principally in their being incapable of unrolling; and, in some cases, in the turns of the spiral fibre being distant or broken, or even, in appearance, branched. 31. In those cases where the turns of the spire actually touch each other, the ducts, which are then called closed, can only be distinguished from spiral vessels by their inability to unrol; while at rest they appear to be absolutely the same. 32. They are found among the woody tissue of herbaceous plants; are abundant in the wood of the higher tribes of cellular plants, such as Ferns and Lycopodiacea, and their ends are often in immediate connection with the loose cellular tissue occupying the extremities of the fibres of the roots. 33 Their functions have not been accurately determined. It is probable that they act as spiral vessels when young; but, it is certain, that they become filled with fluid, as soon as their spires are separated. 34 With the exception of the cellular (18.), nothing is known of the mode of generation of tissue. But, it has been demonstrated that, the cellular is the type of the other kinds, and that from which all the rest are modified; and it is therefore probable that the woody and vascular tissue are generated in a similar way. 35 There are no other elementary forms of tissue. Air-vessels, Reservoirs of oil, Lenticular glands, Proper vessels, are all either distended intercellular passages, or cavities built up with cellular tissue. What are now called vessels of the latex, are apparently intercellular passages. 36. When such cavities are essential to the existence of a species, they are formed by a regular arrangement of cellular tissue in a definite and unvarying figure; Ex. Water-plants. When they are not essential to the existence of a species, they are mere irregular distensions or lacerations of the tissue; Ex. Pith of the Walnut Tree. 37. All these forms of tissue are enclosed within a skin called the cuticle. 38. The CUTICLE is an external layer of parenchyma, the vesicles of which are compressed, and in a firm state of cohesion. 39. The spaces seen upon the cuticle, when examined by a microscope, represent these vesicles. 40. It is, therefore, not a peculiar membrane, but a form of cellular tissue. 41. It is spread over all the parts of plants which are exposed to air, except the stigma (345.). 41. a. It is not found upon parts habitually living under water. 41. b. It is itself protected in many cases, by an extremely thin pellicle, which is apparently inorganic and homogeneous. 42. The mass of cellular tissue lying beneath the cuticle of the bark is called the epidermis. 43. The cuticle is often furnished with stomates. 44. STOMATES are oval spaces lying between the sides of the cells, opening into intercellular cavities in the subjacent tissue, and bordered by a limb. 45. This appearance of a limb is owing to the juxtaposition of two or more elastic vesicles, closing up or opening the aperture which they form, according to circumstances. 46. Stomates are found abundantly upon leaves, particularly on the lower surface of those organs; occasionally upon all parts that are modifications of leaves, especially such as are of a leafy texture; and on the stem. 47. Stomates have not been found upon the roots, nor on colourless parasitical plants, nor the submersed parts of plants, nor on cellular plants destitute of ducts; they are, moreover, rare, or altogether absent in succulent parts. 48. It frequently happens, that they are so incompletely formed, as to be either altogether incapable of action, or to act in a very imperfect manner; as in succulent plants. 49. The function of stomates is to regulate evaporation and respiration. It has been thought, that the former function, in particular, is that for which they are destined; and, that the cause of certain parts becoming succulent, is the absence of stomates, in sufficient numbers to carry off the watery part of the sap. But some succulent plants have more stomates than ordinary plants, so that this opinion requires reconsideration. II. COMPOUND ORGANS. 50. From peculiar combinations of the elementary organs are formed the compound organs. 51. The compound organs are the axis (52.) and its appendages (158.). 52. The AXIS may be compared to the vertebral column of animals. 53. It is formed by the developement of a root in one direction, and of an embryo, or of a leaf-bud in the opposite direction. 54. An embryo is a young plant, produced by the agency of sexes, and developed within a seed. 55. A leaf-bud is a young plant, produced without the agency of sexes, enclosed within rudimentary leaves called scales, and developed on a stem, 56. Seeds propagate the species. 57. Leaf-buds propagate the individual. 58. All the phenomena connected with the growth of plants are caused by an inherent vital action. 59. When the vital action of a seed or bud is excited, the tissue developes in three directions, upwards, downwards, and horizontally. 60. That part which developes downwards is called the descending axis or root; that upwards, the ascending axis or stem; that horizontally the medullary system; and the part from which these two axes start is called the collet or neck. 61 This elongation in three directions takes place simultaneously; hence it follows that all plants must necessarily have an ascending and descending axis, or a stem and root, and a medullary system. 62. The only apparent exceptions to this are vesicular Algæ. III. ROOT. 63. The root is formed by the descending and dividing fibres of the stem. 64. Anatomically it differs from the stem in the absence of spiral vessels (23.), of pith (15.), (in Dicotyledons), of buds, and of stomates (44.). 64. a. Although the root has no distinct pith in Dicotyledons, yet it possesses a distinct medullary system. 65. The functions of the root are to fix plants in the earth, and to absorb nutriment from it. They are also supposed to consist in discharging from the system, whatever secretions injurious to itself a plant may form during its growth. So that some plants, at least, not only deprive the earth of nutritious matter, but deteriorate it by the deleterious matter they deposit there. 66. This absorption takes place almost exclusively by the extremities called spongelets, which consist of a lax coating of cellular tissue lying upon a concentric layer of woody tissue, in the midst of which is often placed a bundle of ducts. |