Botany

Botany, from the Greek botane, a herb, is that division of biology (q.v.) which deais with plants.

In endeavouring, therefore, to define the province of botany as a science, we have to attempt to distinguish the essentials of plant-life from those of animal-life. There is, however, no recognisable line of demarcation between what are sometimes called the two kingdoms of organic nature. Like animals, plants consist largely of protoplasm (q.v.); but most plants differ from most animals in containing relatively less of this substance in an unaltered form and, as a consequence, less nitrogen. Whilst most animals are mainly built up of numerous cells not enclosed in any definite membranes, known as plastids (q.v.), most plants are made up of cells each enclosed in a definite cell-wall composed of cellulose (q.v.), a comparatively simple non-nitrogenous compound. Tha green colouring matter known as chlorophyll (q.v.), though present in most plants, is absent in fungi and some others, whilst it occurs in a considerable number of the lower animals, so that it is not distinctive, and the same must be said of both starch and cellulose. Nor is there any universal physiological distinction. Motion, characteristic of most animals, at least at some period in their lives, occurs in many of the lower plants, and though muscle and nerve, those highly specialised organs of motion, are confined to animals, they do not occur in all animals. The respiration (q.v.) of plants and animals only differs in amount, plants in this respect rather resembling cold-blooded animals; but in the case of green plants in daylight the effect of respiration upon the air is masked by the far more active function of the chlorophyll. This chlorophyllian function, as it is called, consists in the taking in of considerable volumes of carbon-dioxide from the air and the giving out of proportionately large volumes of oxygen. It is a purely nutritive, not a respiratory act, and occurs also in green animals. The chief contrast between plants and animals is undoubtedly in the nature of their food. Plants take in liquid food, generally by roots from the soil, and gaseous food, mostly by their leaves, from the air, this food being inorganic and being built up in the plant into organic compounds. Animals take in solid food, but require it to consist of organic compounds. The exceptions to this rule are the insectivorous plants (q.v.) that digest solid organic food; fungi (q.v.), which cannot construct starch or sugar with the carbon dioxide of the air; some parasitic plants; some that are saprophytic, living upon decaying organic matter; and the green animals already mentioned.

We may define a plant as a living being of one or more cells, or partly of structures formed from cells, these cells being surrounded by a cellulose wall, the plant usually containing chlorophyll, subsisting upon inorganic food and not possessing the power of motion.

Botany may be divided into Pure, Mixed and Applied. Pure Botany, the study of plants in themselves, can be considered under the two aspects of anatomy, or structure, and physiology, or function. Anatomy is perhaps most conveniently divided into histology, the science of tissues or of microscopic structure, and organography, that of external form.

Though we cannot here enter into the details of the science, we may state some of the leading facts under each subdivision. Though most plants are made up of numerous cells, this description is inapplicable to others, to which the name unicellular is commonly applied. These lowest forms, whether fungal, such as the Myxomycetes (q.v.) and Schizophyta, or algal, such as Caulerpa, in which there is apparently a distinct root, stem and leaf, have no internal partitions of cellulose. Most plants, however, not only originate in a single egg-cell, ovum or oospore (q.v.), but, by its repeated division, become multicellular; and, at an early stage in their development, differentiation takes place, groups of similar cells forming tissues and performing special functions. Thus structural differentiation is accompanied by physiological division of labour and there is an intimate connection between histology and function. The whole body of the embryo, or young plant, in its earlier stages, or the growing point of the root or stem of one of the higher plants, consists of a tissue of small, rounded cells, known as parenchyma, with thin walls, filled with protoplasm and thus capable of cell-division, or merismatic. An external cell-layer or epidermis of tubular cells without protoplasm is commonly soon differentiated, and in some cases also a central bundle of elongated cells (prosenchyma), some of which may become fused together by the loss of their transverse partitions into vessels. The outer cell-walls in aerial structures commonly become corky or cuticularised, thus serving to check transpiration or decay from surrounding damp; whilst between certain superficial cells (guard-cells) are adjustable openings regulating transpiration. The vessels just mentioned are essentially a conducting-tissue conveying the liquid food, their walls being generally strengthened against collapse by internal thickening-bands of cellulose. Other tissues, of which wood is the best-known example, are termed mechanical, as adding to the rigidity of such structures as a stem. In a leaf, besides the epidermis with its transpiration-pores or stomata, we have (i) more rigid veins, made up of tough but flexible bast-fibres (mechanical) and spirally-thickened vessels (conducting); (ii) green cells so closely packed below the upper epidermis as to be called palisade-cells, specially adapted to assimilation, the building up of organic compounds from atmospheric carbon; and (iii) loosely-arranged cells below these, giving the paler green colour to the underside of the leaf, with large intercellular spaces communicating with the lower stomata, the transpiration-tissue.

As in the lowest plants or in the earliest stages of higher plants there is no histological distinction between such tissues as these, so too there is little or no distinction in external form or structure between various parts such as root, stem and leaf. Some of the larger sea-weeds, for example, may have root-like holdfasts, rounded stem-like parts, and others flattened and more leaf-like; but the one passes into the other with no articulation nor any difference in internal structure. Such an indeterminate structure is termed a thallus. and the plants which exhibit them, the Algae and Fungi, are called Thallophyta.

Mosses are the lowliest plants in which we can be said to have a true distinction between the stem as an axis and the leaf as a distinct lateral appendage to it; whilst not until we ascend to the still higher grade of the ferns do we meet with the root as a true axial absorbent organ. The parts of a plant considered from the physiological point of view of what function they perform are termed organs. From a purely anatomical point of view they may all be shown to result from the modification of a small number of primitive structures known as members, of which the chief are the thallus, axis, leaf and hair, sometimes termed respectively thallome, caulome, phyllome and trichome. Whilst parts performing the same function are said to be analogous, those referable to the same structural type or member are said to be homologous. As it has been found necessary to base our system of classification upon structure rather than upon function, the study of homologies becomes of extreme importance. The spines of the blackthorn, for instance, are the ends of short branches; those of the Robinia, parts of the leaf (stipules), and the prickles of the rose, distinct superficial structures. The three structures are merely analogous. So too, whilst all tendrils are analogous, some, such as those of the vine, are stem-structures, others, such as those of the pea, are homologous to leaves. The term Organography is generally restricted to the description of the external forms of the parts of plants in general, the comparative study of their development (embryology) under certain general laws of form being distinguished as morphology (q.v.). Closely connected with organography are the rules and terminology employed in the scientific description of plants, the test of which is that an artist understanding the terms should be able to draw the plant from the description. This is called Descriptive Botany.

As our classification of plants depends upon structure, whilst organography deals with the structure of plants generally, there is a distinct department of anatomy known as Special Anatomy, which treats of those structures peculiar to each group. The rules for the classification of plants, or by some writers, the classification itself, are termed Taxonomy, closely connected with which are the rules of Nomenclature, or naming plants. As to the former we can only mention here that "artificial" systems of classification, such as that of Linnaeus (q.v.), based upon one set of characters, are being gradually superseded by an attempt to reconstruct the pedigree of the vegetable kingdom in a "natural system," taking all structural characters into account. As to nomenclature, the main rules are, that every plant has two names, one generic, which it may share with other allied forms, and the other specific, peculiar to one form; and that the first name given to any species in its correct genus in, or after the publication of, Linnseus's Species Plantarum, is that by which it ought to be known.

Passing on to Physiology, the second main division of Pure Botany, we may remark that the functions of the various parts of plants are almost all of them either nutritive or reproductive, the functions of relation, such as motion, sensation, and the special senses, which are so important in the higher animals, being hardly represented among vegetables. The sensitive hairs on the leaf of the Venus' Fly-trap are one of the most strikingly exceptional cases. Plant nutrition can perhaps be best understood by first considering the life of the individual cell (q.v.), after which the action of root and leaf as feeding organs and of the stem as an organ of food-transfer can be considered. Much light is thrown upon this study by organic chemistry (q.v.), the composition of the soil, what different species remove from it, and the composition of the plants themselves and their various organs at various stages of development being most important. Experiments in water-culture, or the growth of plants in solutions of known composition, have done much to show what chemical elements are, and what are not, essential to the life of the plant, substances physiologically useless being commonly taken in by roots. Whilst anatomy is a purely observational study, physiology may be largely experimental in all its departments, the placing the plant under known artificial conditions often explaining functions more clearly than mere observation of the same plant in a natural state can do.

As the result of nutrition, growth is naturally the next subject of study, its rate, direction, and modification by external agencies being the chief heads under which it is considered. Knight's machine, a revolving wheel, is a simple demonstration of the law of "geotropism," that roots grow towards and stems away from the centre of gravity; and it is important "to bear in mind that though moisture, oxygen, and a certain warmth are necessary for growth, and light is necessary for the assimilation of inorganic carbon, light generally retards growth. Thus stems are generally heliotropic, bending towards the light, because their illuminated side grows more slowly.

The movements of plants are partly connected with their unequal growth, as in the unfolding of buds and the "nutation" or nodding of leaves and shoots; and partly with reproduction, the latter class of movements being mostly "irritable," or acting in response to a stimulus and not spontaneously.

Reproduction is either vegetative, as by bulbils, offsets, or runners, or sexual. The former is simply the discontinuous growth of one individual, so that the offspring precisely resembles its one parent. Sexuality, the fertilisation of an ovum or germ cell by a sperm cell, brought about by the most varied means, introducing the fluctuating influence of two parents, brings about the phenomena of variation among seedlings. In the lowest plants sexuality does not seem to have been attained, reproduction taking place simply by fission or bi-partition. Slightly higher in the series we have conjugation, the union, as in Mucor and Spirogyra, of two similar cells. In the bladder-wrack sea-weed, and apparently in most higher plants, we get the union of a relatively large germ-cell with numerous smaller sperm-cells. In several large groups of plants (Cryptogamia) these sperm-cells are detached portions of protoplasm, either free-swimming ciliated "antherozoids" (q.v.), or non-ciliate "spermatia." In flowering plants the male element is merely the formless protoplasm within a "pollen-tube" emitted by a "pollen-grain," which becomes detached from the male organ or "stamen." Lastly, we have many large groups, especially among fungi, in which sex seems to have been lost, some sexual organs still remaining. [Apogamy.] In connection with this subject we have to consider the various agencies by which the pollen-grains are conveyed to the female organ. These are chiefly wind and insects, and many subordinate parts of the higher plants, constituting the "flower," are specially adapted to secure their action. Thus wind - pollinated plants often flower when bare of leaves, having pendulous catkins of inconspicuous flowers with exposed stamens, yielding abundant fine-grained, round, and smooth pollen, and their stigmas, the sticky receptive surfaces of the female organs, feathery. Insect-pollinated flowers, on the other hand, are commonly conspicuous, bright-coloured or strongly scented, secreting honey from glands indicated by dots, lines, or other variegations, and producing large pollen-grains, the surfaces of which are commonly furnished with spines, knobs, or ridges, by which they are entangled in the hairs on the insect's body. Some plants again are adapted for self-pollination, and may even have some flowers, as in the violet, cleistogamous, i.e. fertilised without unfolding. Closely connected with fertilisation are the questions of hybridism, the possibility in many cases of obtaining fertile seed from the pollination of a flower by pollen from a distinct species. Rhododendron and Azalea and many genera of orchids even produce bigeners or bigeneric hybrids, in which the parent species belong to two different genera, and the hybrid seedling may even be fertilised by a third genus, and so on. When, as the result of fertilisation, the ovule, or immature seed of a flowering plant, has developed into a seed with its store of food-substances, either in the embryo or in the surrounding tissue or albumen, the questions of seed-dispersal and of germination arise. Seeds are commonly furnished with a tough, impermeable outer coat, resisting even the action of sea-water or digestive acids, and checking premature germination. If small, they may be carried by wind, and they may have tufts of hair, as in the willow, or wing-like membranes, as in the pine. The variously formed fruits in which they are enclosed may, if dry, be small and be similarly provided with a pappus of hairs, as in the thistles, or with wings, as in the sycamore; or may adhere by hooks or bristles to the wool or fur of animals; or may burst elastically, as in the balsam; or, if succulent, may attract birds or other animals, and be eaten, whilst the seed they enclose is rejected undigested.

In the ripe seed there is generally some store of starch, oil, aleurone (q.v.), or other food material. Under the suitable conditions of warmth, moisture and oxygen, the seed absorbs water and swells, and fermentative changes, such as the conversion of insoluble starch into soluble malt-sugar, take place within, and the radicle, or primary root of the embryo, bursts its way out, followed immediately in some cases by the cotyledons, or embryonic leaves, and in others by the plumule or primary bud of the axis.

Anatomy and physiology thus dealing with the entire structure and life history of plants, Mixed Botany, in which the science is mainly subsidiary to geography and geology, deals with the distribution of plants in space and time. In Palaeozoic rocks the only known plant-remains are Cryptogamia (q.v.), or flowerless plants, and Gymnospermia(q.v.), or cone-bearers, Angiospermia, or ordinary flowering-plants, apparently originating in the Secondary period, not till the close of which did Dicotyledons (q.v.) become numerous. This branch of botany is termed Paleobotany or Palaeophytology.

The science of botany is applied to various arts, plants and vegetable products being put to so many and so various uses. Thus Applied Botany is practically co-extensive with Economic Botany or Vegetable Technology. Vegetable products include food substances for men and animals, materia medica, oils, gums, dyes, tanning materials, fibres, paper-materials, timbers and others, each class forming the subject of a separate department of the study. Materia medica or pharmaceutical botany has been most carefully investigated from the points of view of both the chemist and the systematist. The study of useful plants when alive, their cultivation, diseases, preparation, etc., forms the subject of agricultural, horticultural and arboricultural botany.