THE FAMILIES OF PLANTS

THAT plants fall naturally into groups of similar kinds is a fairly obvious fact. We all know quite well what we mean when we speak of a ‘tree,’ and do not for one moment suppose it to be the same thing as a fern or a seaweed. Moreover, if we are interested in such matters, we soon learn to distinguish in some detail between the kinds, or ‘species,’ of trees as such, so that we talk of oaks, beeches, elms, poplars, pines, and so forth. Also, we go on to trace numberless resemblances and differences between the various species; as, for example, that the oak casts its leaves at the approach of winter, while the pine does not; or, that the fruit of the oak is a nut-like acorn, while that of the pine is a scaly cone. In other words, we find by observation that plants may be arranged for purposes of study into larger or smaller groups of similar kinds—the members of one such group, despite more or less evident divergencies among themselves, being liker to one another than to the kinds which we place in other groups. Thus we arrive at the notion of family relationship among plants.

This is really the root idea in the theory of evolution. If all the existing species of plants have ‘evolved,’ as Charles Darwin suggested that they have, by a process of natural selection from a few primitive types—perhaps even from one original, then we should expect to discover some evidence of this by comparing the structural peculiarities of one plant with those of others, and by noting the changes which take place as an individual plant grows from its seed to maturity. Geology, too, will be likely to furnish clues to what has taken place; and may perhaps show conclusively, by producing fossils, that plants which according to our theory ought to have existed at some period of life’s history did actually exist, but have since become extinct. These, in short, were the kinds of investigation, unremittingly pursued over a long period of years and in many directions, which led Darwin to publish (1859) his famous book, On The Origin of Species.

Prior to this, the classification of plants by botanists had been more or less artificial. It is true that the great Swedish botanist, Linnaeus, based his system on the ‘natural ‘affinities of plants as revealed by a comparison of their essential—or reproductive—organs. But because (to quote his own words) Linnaeus believed that ‘there are just as many species as in the beginning the Infinite Being created,’ he had no conception that the resemblances which he noted were not merely coincidental, but were indications of descent from a common ancestry. The later view was made possible by the work of Darwin, and it has enabled modern botanists to realise that the ideal aim of classification is to plot out a huge pedigree, or;’ family tree,’ of plant life.

EVERY WEED HAS ITS RELATIVES : SOME SURPRISING KINSHIPS NOWADAYS, therefore, this family relationship of plants is given practical expression in books and museums by arranging the various species in groups according to their true kinship, as far as this is known. ‘Kinship ‘is not at all the same thing as mere superficial similarity. Because two people happen to resemble one another in height, weight, features and character, it by no means follows that they are near relatives. To make this point clear, consider for a moment two of our native wild plants, both known popularly by the name ‘Pennywort,’ in reference to the coin-like roundness of their leaves.

The Wall Pennywort grows in dry, sunny situations, more especially in the West of England, where it often roots in the scanty soil accumulated in the fissures of rocks and stonework. Expert examination of its specific characters, more especially those of its flowers, indicates that its nearest relatives are the Stonecrops and Mouse-leeks, in which family —or ‘order ‘—it is accordingly placed, in.

The Marsh Pennywort, which creeps among moss and grass in marshes and bogs, has almost identical leaves. But the structure and arrangement of its tiny flowers (they require a close search to be detected) bring to light the surprising fact that this modest plant is really a member of the Umbelliferas— the clan which comprises the stately Hemlocks and the Hog-weeds, to say nothing of numerous species useful to man, of which the Carrot, the Celery, and the Caraway are three examples.

THE CLUES TO A PLANT’S IDENTITY

A BRIEF survey of the more important families of plants is all that can be given here; but if the reader bears in mind this underlying fact of real relationship, as distinct from mere ‘appearances ‘(which are proverbially deceptive), he will not fail to grasp the principle from which the modern systematic botanist takes his cue. The relative complexity of structure in the plant-body, and the character of the reproductive organs—these are the points to which attention must chiefly be directed.

Starting, then, with a general survey, we find that living plants may be separated into five main groups or divisions, viz.:

(1)Plants known collectively as ‘Alga;,’ many of them minute and single-celled, but comprising also the larger ‘seaweeds; ‘fungi of all sorts, including the growths called ‘moulds.’ (Thallophyta.)

(2)Liverworts and mosses. (Bryophyta.)

(3)Club-mosses, horse-tails and ferns. (Ptcridophyta.)

(4)Coniferous trees and their allies. (Gymnosperma;.)

(5)Plants which in general bear conspicuous ‘flowers,’ I.e. groups of ‘male ‘and ‘female ‘reproductive organs. often surrounded by a whorl or whorls of gaily coloured petals, and perhaps associated with nectar-secreting glands. (Angiospenrue.)

Charles darwin

Charles darwin (Photo credit: Wikipedia)

The scientific names of these divisions are given in brackets, so that the reader may ignore or memorise them at discretion. Their chief importance consists in the fact that they are portmanteau words which compress into small compass information concerning some outstanding characteristic common to members of the particular group. Thus, the ‘Thallophyta ‘are so called because the individual plant, when many-celled, is what botanists term a ‘thallus,’ that is, a simple vegetative body without clearly distinguishable organs, such as stems and leaves.

THE ROOT-STOCK OF PLANT PEDIGREE

IT must be admitted, however, that this first division of the vegetable kingdom is a somewhat mixed assemblage. It includes, for example, the organisms generally spoken of as ‘bacteria ‘and ‘slime-fungi,’ concerning whose true status so much doubt exists that competent authorities still question whether they are really plants at all, or even lowly

animals, but perhaps only ‘living things.’ Be this as it may they undoubtedly differ widely in many respects from all other known plants.

The bacteria are microscopically small, single-celled creatures which, in favourable circumstances, increase rapidly by splitting or ‘fission.’ Some live in the soil, where they often play an important part in preparing food-material for the higher plants. They convert, for instance, nitrogenous substances into forms which can be readily assimilated by green vegetation. Certain flowering plants, notably those of the pea-and-bcan family, have entered into a kind of partnership with those species of soil-frequenting bacteria which are especially serviceable to them in this respect, and not only tolerate thriving colonies of these minute purveyors (‘nitrogen-fixers,’ as they are called) in close association with their roots, but may fail to flourish in their absence.

Other bacteria live in the tissues of higher organisms, both plants and animals, sometimes harmlessly—or even helpfullv —as in the case of those which normally inhabit the food-

canal of the healthy human body; but often injuriously, since specific forms of bacteria are known to be the active agents in many infectious diseases, including cholera, diphtheria and typhoid.

In unfavourable circumstances, when their usual mode of reproduction by-fission becomes impracticable, bacteria adopt the method of spore-formation. A ‘spore ‘is the technical name for a single cell capable of giving rise without any sexual process to a new individual. Spores lie dormant for a longer or shorter period until the conditions essential for active life again present themselves.

MASSES OF SLIME THAT CREEP FOR FOOD

THIS simple method of propagation also characterises the Slime-fungi—the spore-containing capsules, or ‘sporangia,’ of o.ie species, commonly found among the refuse of tanneries, being the so-called ‘flowers of tan.’ The capsules of other species occur on dead twigs and decaying leaves in woods, and are rather attractive objects when viewed through a pocket lens. Not so the Slime-fungus in its active state, for it has much in common with certain lowly animals. Generally speaking, it is an almost formless mass of slimy protoplasm, devoid of definite cell-structure or special tissues; but there are numerous small cavities, or ‘vacuoles,’ in the living jelly, some of which contain water or gas, others food-material in process of digestion.

Perhaps the most curious point about this organism is that it travels by a spontaneous movement, and thus brings itself into contact with the food-substances on which it lives. To quote Wells and Huxley, ‘It will turn aside from its course to flow over an attractive lump of food. In most cases only dead and decaying things are taken into its shapeless interior to be digested, but there are species which feed on living vegetable prey—on fungi, for example.’

THE CAUSE OF ‘CLUB-ROOT’

THE extremely infectious disease called by farmers and gardeners ‘club-root ‘or ‘finger-and-toe ‘—it attacks plants of the cabbage family—is caused by a minute slime-fungus which is of microscopic size. Some innocuous kinds of slime fungi, however, grow to be conspicuous masses, such as those which may be found in the cavities of rotting logs, or on heaps of decaying matter in damp surroundings.

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