Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/224

212 212 HORTICULTURE [GENERAL there is a generally recognized quasi-antagonism between the vegetative and reproductive processes, so that, other things being equal, anything that checks the one helps forward the other. Watering. So far as practical gardening is concerned, feeding by the roots after they have been placed in suitable soil is confined principally to the administration of water and, under certain circumstances, of liquid manure ; and no operations demand more judicious management. The amount of water required, and the times when it should be applied, vary greatly according to the kind of plant and the object for which it is grown, the season, the supply of heat and light, and numerous other conditions, the influence of which is to be learnt by experience only. The same may be said with respect to the appli cation of manures. The watering of pot-plants requires especial care. Water should as a rule be used at a temperature not lower than that of the surrounding atmosphere, and preferably after exposure for some time to the air. Bottom-Heat, The &quot; optimum &quot; temperature, or that best suited to promote the general activity of roots, and, indeed, of all vegetable organs, necessarily varies very much with the nature of the plant, and the circumstances in which it is placed, and is ascertained by practical experience. Artificial heat applied to the roots, called by gardeners &quot; bottom-heat,&quot; is supplied by fermenting materials, dung, tan, flues, or hot-water pipes. In some cases solar heat is as it were entrapped by placing beneath the roots substances such as bricks, the heat previously absorbed by which is slowly radiated. In winter the temperature of the soil, out of doors, beyond a certain depth is usually higher than that of the atmosphere, so that the roots are in a warmer and more uniform medium than are the upper parts of the plant. Often the escape of heat from the soil is prevented by &quot; mulching,&quot; i.e., by depositing on it a thick layer of litter, straw, dead leaves, and the like. The Stem and its subdivisions or branches lengthen, not only near the tip, but also lower down, by intercalary growth. They upraise to the light and air the leaves and flowers, and serve as channels for the passage to them of fluids from the roots, and they act as reservoirs for nutritive substances. Their functions in annual plants cease after the ripening of the seed, whilst in plants of longer duration layer after layer of strong woody tissue is formed, which enables them to bear the strains which the weight of foliage and the exposure to wind, &c., entail. The gardener aims usually at producing stout, robust, short-jointed stems, instead of long lanky growths defective in woody tissue. To secure these conditions free exposure to light and air is requisite, but in the case of coppice woods, or where long straight spars are needed by the forester, plants are allowed to grow thickly so as to ensure development in an upward rather than in a lateral direction. This and like matters will, however, be more fitly considered in dealing hereafter with the buds and their treatment. Leaves. The work of the leaves may briefly be stated to consist in the processes of nutrition and of respiration. Nutrition by the leaves includes the inhalation of air, and the combination, under the influence of light and in the presence of chlorophyll, of carbon from the carbonic acid gas in the air with hydrogen from the watery vapour it contains, oxygen being exhaled. There is also a process of true respiration, in which atmospheric oxygen unites with a portion of the carbon in the plant, and is evolved us carbonic acid gas. Syringing, &amp;lt;kc. In certain circumstances water is ab sorbed by the surface of leaves, especially when the supply of moisture at the root is defective, and when by too long exposure to drought the watery constituents of the plant have evaporated. A certain amount of evaporation of superfluous watery fluid or vapour is a necessary accom paniment of nutrition, but this may easily become exces sive, especially where the plant cannot readily recoup itself. In these circumstances such operations as &quot;syringing&quot; and &quot;damping down&quot; are of special value. Evaporating basins or tanks in houses for orchid and other plants are beneficial for like reasons. Following Boussingault and Henslow, by whom the absorption of water by leaves has been proved, we may sum up the advantages of syringing as follows : it washes off dust and insects from the leaves, and by moistening the cuticle promotes res piration and the absorption of water ; it checks loss by transpiration, and so enables terminal shoots and young leaves to receive a sufficiency of sap from the stem ; it keeps the air cool by evaporation ; and lastly, as moisture is actually imbibed by the green parts of plants, it helps to compensate for any loss from within, and thus supplements root absorption. In accordance with the facts just cited it is found that the preservation of cut flowers is promoted by inserting some of their leaves as well as their stalks in water. By cutting an herbaceous stem under water, so that the severed end is never exposed to the air, withering can to a large extent be prevented ; and a bouquet may be kept fresh for a long time either by immersing the whole in water, or by simply covering the vase of water in which it is placed with a bell-glass. In the case of &quot;cuttings&quot; exces sive transpiration is obviated by means of bell-glasses and by shading, and sometimes by burying a portion of the cutting with a leaf attached. Carnivorous Plants. Before leaving the subject of nutrition by leaves, reference may be made to the so-called carnivorous plants, e.g., Drosera. Substances, particularly such as contain nitrogen, as insects or fragments of meat, when brought into contact with the surface of the leaves, or with certain glands on the leaves, become dissolved by the agency of a digestive ferment secreted therein, are then absorbed and serve as nutriment. See INSECTIVOROUS PLANTS. Respiration, already alluded to, is not directly connected with exposure to light, since it goes on by night as freely as by day. It is a process requisite for the health of the plant, and contributes to maintain its heat, to perfect its structure, to eliminate some of its secretions, and to destroy effete or impure matters. Chlorophyll, according to Prings- heim, acts as a regulator of the respiration of plants by absorbing some of the luminous rays, and thereby favour ing the function of assimilation. As a result of the processes carried on in the leaves under the influence of light, many of the secretions, as starch, sugar, oils, and colouring and odoriferous matters are formed. These either at once subserve the nutrition of the plant or are stored in its tissues, as in the case of tubers and many seeds. Usually before it can be rendered available as nourishment the stored matter has to undergo a change from a more or less insoluble to a soluble form. The changes which they undergo within the plant require very careful study, and indeed constitute a department of physiology still very greatly in need of elucidation. Pringsheim s recent researches on the action of light on chlorophyll, prosecuted with the aid of a small lens and observed under the microscope, bid fair, if confirmed, materially to change the views of chemists as to the pro cesses which go on in leaves as a consequence of exposure to light; but, although they may change or even reverse our notions as to the mode of action of chlorophyll, they will not detract from its importance. Buds. The recognition of the various forms of buds,