PRINCIPLES. ft.1 HORTICULTURE 215 varieties of flowers and fruits have thus originated. The cause of their production is very obscure. In certain instances where plants have been " crossed " or hybridized, perhaps for generations, the phenomenon may be explained on the supposition of a dissociation of previously mixed elements, or of a reversion to some ancestral conditions. Formation of Floivers. Flowers, whether for their own sake or as the necessary precursors of the fruit and seed, are objects of the greatest concern to the gardener. As a rule they are not formed until the plant has arrived at a certain degree of vigour, or until a sufficient supply of nourishment has been stored in the tissues of the plant. The reproductive process of which the formation of the flower is the first stage being an exhaustive one, it is necessary that the plant, as gardeners say, should get " established " before it flowers. Moreover, although the green portions of the flower do indeed perform the same office as the leaves, the more highly coloured and more specialized portions, which are further removed from the typical leaf-form, do not carry on those processes for which the presence of chlorophyll is essential; and the floral organs may, therefore, in a rough sense, be said to be parasitic upon the green parts. A check or arrest of growth in the vegetative organs seems to be a necessary preliminary to the development of the flower. The flower itself is always the modified extremity of a shoot or stalk, which only exceptionally lengthens beyond the flower, as, for example, in " proliferous " roses, See BOTANY, vol. iv. p. 119, fig. 145. A diminished supply of water at the root is requisite, so as to check energy of growth, or rather to divert it from leaf-making. Partial starvation will sometimes effect this; hence the grafting of free-growing fruit trees upon dwarfing stocks, as before alluded to, and also the "ringing" or girdling of fruit trees, i.e., the removal from the branch of a ring of bark, or the application of a tight cincture, in consequence of which the growth of the fruits above the wound or the obstruction is enhanced. On the same principle the use of small pots to confine the roots, root- pruning and lifting the roots, and exposing them to the svin, as is done in the case of the vine in some countries, are resorted to. A higher temperature, especially with deficiency of moisture, will tend to throw a plant into a flowering condition. This is exemplified by the fact that the temperature of the climate of Great Britain is too low for the flowering, though sufficiently high for the growth of many plants. Thus the Jerusalem artichoke, though able to produce stems and tubers abundantly, only flowers in exceptionally hot seasons. Forcing. The operation of forcing is based upon the facts just mentioned. By subjecting a plant to a gradually increasing temperature, and supplying water in proportion, its growth may be accelerated ; its season of development maybe, as it were, anticipated; it is roused from a dormant to an active state. Forcing therefore demands the most careful adjustment of temperature and supplies of moisture and light. Deficiency of light is less injurious than might at first be expected, because the plant to be forced has stored up in its tissues, and available for use, a reserve stock of material formed through the agency of light in former seasons. The intensity of the colour of flowers and the richness of flavour of fruit are, however, deficient where tli ere is feebleness of light. Recent experiments of Dr Siemens have shown that the gardener may avail him self of the electric light, which is proved to exercise on chlorophyll the same kind of influence as do the solar rays, and that he may thus supply the deficiencies of natural illu mination. The employment of that light for forcing purposes would seem to be at present a question of expense. The great advantage hitherto obtained from its use has consisted in the rapidity with which flowers have been formed and fruits ripened under its influence, circumstances which go towards compensating for the extra cost of production. Double Flowers. The taste of the day demands that "double flowers" should be largely grown. Though in some instances, as in hyacinths, they are decidedly less beautiful than single ones, they always present the advan tage of being less evanescent. Under the vague term " double " many very different morphological changes are included. The flower of a double dahlia, e.g., offers a totally different condition of structure from that of a rose or a hyacinth. The double poinsettia, again, owes its so- called double condition merely to the increased number of its scarlet involucral leaves, which are not parts of the flower at all. It is reasonable, therefore, to infer that the causes leading to the production of double flowers are varied. A good deal of difference of opinion exists as to whether they are the result of arrested growth or of exuberant development, and accordingly whether restricted food or abundant supplies of nourishment are the more necessary for their production. It must suffice here to say that double flowers are most commonly the result of the substi tution of brightly-coloured petals for stamens or pistils or both, and that a perfectly double flower where all the stamens and pistils are thus metamorphosed is necessarily barren. Such a plant must needs be propagated by cuttings. It rarely happens, however, that the change is quite com plete throughout the flower, and so a few seeds may be formed, some of which may be expected to reproduce the double-blossomed plants. By continuous selection of seed from the best varieties, and " roguing " or eliminating plants of the ordinary type, a " strain " or race of double flowers is gradually produced. Formation of Seed Fertilization. In fertilization the influence in flowering plants of the sperm-cell, or its contents upon the germ-cell (see BOTANY, vol. iv. 147, and BIOLOGY, vol. iii. 695) there are many circumstances of importance horticulturally, to which therefore brief reference must be made. Flowers, generally speaking, are either self-fertilized, cross-fertilized, or hybridized. Self-fertilization occurs when the pollen of a given flower affects the germ- cell of the same individual flower. Such a flower is hermaphrodite functionallyas well as structurally. In self-fertilizing flowers the structure is such that the pollen inevitably comes in contact with the stigma ; but fertilization is also dependent on the simultaneous maturity of pollen . and stigma. Cross-fertilization varies both in manner and degree. In the simplest instances the pollen of one flower fertilizes the ovules of another on the same plant, owing to the stamens arriving at maturity in any one flower earlier or later than the pistils. Such flowers though structurally hermaphrodite are, at any given time, function ally unisexual. In many plants a polymorphic condition occurs : thus, in the same species of primrose some flowers have the stamens short, and within the tube of the corolla, with the style projecting beyond the mouth, giving the appearance termed "pin-eyed," while others, known as " thrum-eyed," present exactly reverse conditions of stamens and style. In the common loose-strife, LytJirum Salicaria, the stamens are of three lengths, and the styles differ corre spondingly. In such di- or tri-morphic flowers, as Mr Dar win s experiments have shown, the most complete fertility occurs when a cross is effected between a flower having short stamens and one with short styles. It is asserted that, not only is such a union more fertile than when pollen from a short stamen is placed on a long style, or vice versa, but the seedling plants are as a rule more vigorous. Cross-fertilization must of necessity occur when the flowers are structurally unisexual, as in the hazel, in
which the male and female flowers are monoecious, or
