The ordinary soils of our fields may be defined as a mixture of sand, clay, and humus. A soil which is too rich, or too poor, in any one of the three will support plant life with difficulty.
The roots of plants require also a due amount of both water and air if they are to fulfil their functions adequately. An examination of the minute structure of the soil shows that it consists of angular particles of very various size—the larger ones classed as sand and consisting largely of silica; the smaller, which decrease in size beyond the limits of microscopic vision, mainly of clay (silicates) and humus. A film of water clings round each particle, and between the particles the chinks are filled with air. For healthy plant growth a nice balance between these constituents is required. Should sand be in excess, the soil is impoverished, since silica contains no nutriment, and it is rendered too dry, as on account of the relatively small surface of the sand grains in proportion to their mass it retains but little water. Should there be too little sand, percolation of air and water is hampered; the soil tends to become water-logged and badly aerated, and turns sour. Should humus be absent, the nitrogen-producing bacteria cease their activities and the soil is sterile, as may be tested by digging up some subsoil, or soil from the deeper levels to which roots or other organic matter have never penetrated. An excess of humus, on the other hand, results in the accumulation of acid products inimical to bacterial growth: in consequence decay is arrested, and a mass of plant débris forms, highly charged (for humus is very spongy) with acid water and badly aerated, which is unsuitable for vegetable growth: we may study an extreme case of such conditions in our peat bogs. Should water be in excess in soils, air is forced out in proportion, and the roots cannot breathe. Too much air means a corresponding diminution of water, and the plants suffer from drought.
“The soil is not merely a reservoir for the mineral nutrients of plants, but is the seat of complex physical, chemical, and biological actions which directly and indirectly influence soil fertility. These actions are intimately associated with the organic matter of the soil and its bacterial inhabitants. Mineralogy and inorganic chemistry, though helpful, are no longer capable of solving soil problems. Biochemistry and bacteriology, with their modern conceptions of colloids, absorption phenomena, enzymes, oxidizing, reducing, and catalytic actions, etc., are now rapidly extending our knowledge of the soil as a medium for plant growth.”[8]
Such, then, is the nature of the soil in which plants grow, and from which, by means of innumerable elongated cells (the root-hairs) proceeding from near the tips of the roots, food materials dissolved in water are absorbed; these food materials being produced partly by solution of mineral constituents contained in the soil, partly by the action of bacteria in breaking up organic matter. Soil suitable for plant growth may be looked on as consisting of a mineral framework, carrying in its meshes water (about three-tenths of its volume) and air (about one-tenth of its volume); mixed with the mineral particles is humus of varying amount; and supported largely by the humus is a vast population of organisms, both animal and vegetable, from earthworms to bacteria, whose activities are often essential, generally beneficial, and occasionally prejudicial to plant growth.
The root of a young plant grows downward into the soil under the influence of gravity. Its tip, which has to force its way through the rough material of sand and clay, is beautifully protected by a special root-cap, which covers the growing point as with a cushion. The surface of the root-cap is slimy, to aid it in slipping forward, and its cells, which are being worn away constantly, are replaced by the growth of the interior. Should an obstacle such as a pebble be encountered, a root will bend round it and then return to its former direction. Branch roots are given off on all sides at an angle to the main stem, these also tending in a mysterious way, if their course is disturbed by an obstacle, to resume their former direction of growth; the branches again divide, till at length a complicated root-mass is formed, sometimes of great extent, and capable of extracting water from a large volume of soil. Save for continued growth, the roots show little change in comparison with those exhibited by the aerial parts of plants; safely immersed in the soil, they heed not day or night, storm or calm, but steadily pursue their main function of supplying liquid food material to the green parts overhead.
In many instances roots do not accomplish their work single-handed, but only in co-operation with certain lowly organisms; and these cases are so interesting and of so much economic importance that reference should be made to them. The little swellings or tubercles upon the roots of Leguminous plants, such as Clover, are familiar to most of us. These are caused by the stimulation due to colonies of bacteria (Bacillus radicicola), which live in the root-tissues as internal parasites. These bacteria feed on the sap and cell-contents of their host, but they supplement this food-supply by absorbing nitrogen direct from the atmosphere, which the host cannot do, though it can and does use the nitrogenous compounds which the bacteria manufacture. It is a case of symbiosis (see p. 79), each organism supplying food useful to the other; but the significance of the phenomenon is that through this agency nitrogen becomes added to the soil as the plants decay, and increases its fertility; and thus the cultivation of a crop of, say, Lucerne becomes a matter of great economic importance in farming operations, and the presence of Clover in pasture is a source of increasing wealth.
Again, in the roots of most of our forest trees, both hardwoods and conifers, and of many other plants such as the Ericaceæ and Orchidaceæ, the root-hairs are replaced by minute fungi known as mycorhiza, whose branches take on the function of absorption, while the roots in turn absorb the material which the fungus collects. The fungus obtains from the roots a direct and convenient supply of carbohydrates; the host obtains from the fungus a ready supply of salts and of nitrogenous compounds. In the case of the forest trees and some other plants, the fungus forms a close felt around the roots; but in the Heaths, etc., it penetrates the roots, living in the cells and in some instances, as in the Ling (Calluna vulgaris), permeating the whole plant, even to the seed-coat, so that seed and fungus are sown together. Since the higher partner of the symbiosis cannot mature without the lower, this is an obvious advantage to the former, as the two develop together from the commencement of growth. Where the fungus is not present in the seed, the seedling has to rely on its presence in the soil. And so, if we wish to raise any of our common terrestrial Orchids from seed, we try to ensure the presence of the fungus by using soil in which the species has been growing already.
The state of mutual dependence existing between seed plants and mycorhizic fungi sometimes ends in the higher organism ceasing to manufacture its food by means of green leaves, and depending wholly on the lower for its sustenance. This is the condition to which some of our Orchids have come, such as the Bird’s-nest (Neottia Nidus-avis), which does not produce leaves or chlorophyll, but sends up from its fungus-infested roots merely a scaly brown stem topped with brown blossoms, matching curiously the dead leaves among which it grows ([Fig. 31], p. 182).
In contrast to these the case of certain other Orchids may be quoted, which have also lost their leaves, but in a very different manner. In their case the roots, creeping over the bark of trees on which the plants perch as epiphytes, have become green and flattened, like the fronds of some of our native Liverworts; they have assumed the functions of leaves: in them the process of photosynthesis is carried on; and the leaves themselves, thus supplanted, have by degrees disappeared.
Like many other parts of plants, roots are often used for the storage of reserve supplies of food or of water. For this purpose they become much thickened, and this thickening is the most conspicuous change which roots usually undergo. Note the fat roots of many plants which grow in dry or arid places, such as the Sea Holly, Dandelion, and many desert plants and alpines. The thickening is often accompanied by increase in length, as the roots range far in search of water. Another point to notice is that though normally roots differ considerably from their associated stems in general appearance, and also in their minute structure, as in the arrangement of the vascular strands, the two are related. Stem structures are often produced at various points on roots; the suckers sent up by many kinds of trees offer an example. Conversely, roots are readily produced even from the upper portions of many stems—else how could we grow cuttings? Where roots are succulent—that is, when they have a reserve of food stored in them—cuttings of them will conversely produce stems. A classical instance of such interchangeability of function is the young willow which Lindley bent down and buried the top till it rooted; the original roots were then dug up and raised into the air, when they produced leafy branches, and the tree grew upside down henceforth. Underground stems, also, of which there is a great variety, take on many of the characters of roots, and from an examination of a small piece of one it is often difficult to tell whether we are dealing with a root or a stem. The point at which root joins stem is, in fact, in many instances, so far as function is concerned, fixed only so long as the level of the surface remains fixed: we can often alter it by “earthing up” or by stripping away the soil. In Tropical forests, where the air is moist, hot, and still, roots—or branches which serve only as roots—descend through the air from heights almost equalling those to which stems ascend; while, on the other hand, in hot, poorly aerated swamps, roots send up from the mud into the air stem-like structures (pneumatophores) through which they may breathe, as in the case of the Swamp Cypress (Taxodium distichum) of Florida. The primary differences between the two, in fact, do not prevent the one from taking on the general characters of the other, and from functioning as the other, when the environment changes.