216 HORTICULTURE 1 GENERAL separate on the same plant, and in the willow, in which they are dioecious, or on different plants. A. conspicuous example of a dioecious plant is the common aucuba, of which for years only the female plant was known in Britain. When, through the introduction of the male plant from Japan, its fertilization was rendered possible, ripe berries, before unknown, became common ornaments of the shrub. The conveyance of pollen from one flower to another in cross-fertilization is effected naturally by the wind, or by the agency of insects and other creatures. Flowers that require the aid of insects usually offer some attraction to their visitors in the shape of bright colour, fragrance, or sweet juices. The colour and markings of a flower often serve to guide the insects to the honey, in the obtaining of which they are compelled either to remove or to deposit pollen. The reciprocal adaptations of insects and flowers demand attentive observation on the part of the gardener concerned with the growing of grapes, cucumbers, melons, and strawberries, or with the raising of new and improved varieties of plants. Scarcely less remarkable, though not so important in the present connexion, are the means by which the visits of such insects as are useless for the purpose of fertilization, or even injurious to the plant, as preying without advantage to it on its secretions, are prevented or rendered ineffective. In wind-fertilized plants the flowers are comparatively inconspicuous and devoid of much attractions for insects ; and their pollen- cells are smoother and smaller, and better adapted for transport by the wind, than those of insect-fertilized plants, the roughness of which adapt them for attachment to the bodies of insects. Although the general facts with respect to fertilization are as above stated, it must be remembered that probably self-fertilization is not constant in any plant under all circumstances, and that it certainly does sometimes take place in flowers which are usually cross-fertilized. It may be that, while continued self-fertilization ensures the perpetuation of certain qualities, cross-fertilization induces beneficial variation. Some botanists doubt the injurious effects attributed to self-fertilization, and, so long as a plant is healthy, it can be attended with but little dis advantage ; but after a time in any case a cross is probably useful, and sometimes fertility is found to be much greater, and, in rare instances, only possible, when impregnation is effected by pollen not produced by a flower s own stamens. It is very probable that the same flower at certain times and seasons is self-fertilizing, and at others not so. The defects which cause gardeners to speak of certain vines as "shy setters," and of certain strawberries as "blind," may be due either to unsuitable conditions of external temperature, or to the non-accomplishment, from some cause or other, of cross-fertilization. In a vinery or a peach- house it is often good practice at the time of flowering to tap the branches smartly with a stick so as to ensure the dispersal of the pollen. Sometimes more delicate and direct manipulation is required, and the gardener has himself to convey the pollen from one flower to another, for which purpose a small camel s-hair pencil is generally suitable. The degree of fertility varies greatly according to external conditions, the structural and functional arrangements just alluded to, and other causes which may roughly be called constitutional. Thus, it often happens that an apparently very slight change in climate alters the degree of fertility. Certain plants which seem almost sterile with their own pollen become fertile if grafted on some others. In a particular country or at certain seasons one flower will be self-sterile or nearly so, and another just the opposite. The influence of conditions on the formation of " races," and the consequent importance to the horticulturist seeking to obtain new and improved strains of crossing-plants grown in different localities, have been specially insisted on by Darwin. The advantages of this practice are analogous to those accruing from what gardeners call " change of seed," i.e., the sowing of seed or the planting of tubers, say of potatoes, in localities and on soils other than those in which they themselves were produced. Hybridization. Some of the most interesting results and many of the gardener s greatest triumphs have been obtained by hybridization, i.e., the crossing two individuals, not of the same but of two distinct species of plants, as, for instance, two species of rhododendron or two species of orchid. It is obvious that hybridization differs more in degree than in kind from cross-fertilization. The occurrence of hybrids in nature explains the difficulty experienced by botanists in deciding on what is a species, and the widely different limitations of the term adopted by different observers in the case of willows, roses, brambles, &c. The artificial process is practically the same in hybridization as in cross-fertilization, but usually requires more care. To prevent self-fertilization, or the access of insects, it is advis able to remove the stamens and even the corolla from the flower to be impregnated, as its own pollen or that of a flower of the same species is often found to be " prepotent. 1 There are, however, cases, e.g., some passion-flowers and rho dodendrons, in which a flower is more or less sterile with its own, but fertile with foreign pollen, even when this is from a distinct species. It is a singular circumstance that reci procal crosses are not always or even often possible ; thus, one rhododendron may afford pollen perfectly potent on the stigma of another kind, by the pollen of which latter its own stigma is unaffected. With respect to the relations of the hybrid offspring, which partakes sometimes more of the characteristics of the male or pollen parent, sometimes more of those of the female or seed-parent, the opinions of practical experimenters are so diverse that at present no general rule can be established. A valuable essay on the subject is the presidential address read by Mr Anderson- Henry at the annual meeting of the Botanical Society of Edinburgh in 1867. A general resume of the facts will be found in Darwin s Origin of Species, his Variations of Ani mals and Plants under Domestication, and his works on the- fertilization of flowers. See also HYBRIDISM. The object of the hybridizer is to obtain varieties ex hibiting improvements in hardihood, vigour, size, shape, colour, fruitfulness, or other attributes. His success depends not alone on skill and judgment, for some seasons, or days even, are found more propitious than others. Although promiscuous and hap-hazard procedures no doubt meet with a measure of success, the best results are those which are attained by systematic work with a definite aim. To secure early and free-flowering varieties, Mr Henry advises "violent" crosses, i.e., crosses between varieties or species as distantly related as is practicable. Careful experiments are still greatly needed for the elucidation of the mysteries and the development of the resources of hybridization. It is difficult to understand why some very closely-related species, e.g., the apple and pear, the currant and gooseberry, refuse to intercross, while much more remote species, or even members of different genera, can be made to do so, as in the case of the hybrid Philageria (see Gardeners Chronicle, 1872, p. 358), which is the result of a cross between the climbing plant Lapageria rosea and the dwarf bush Pkilesia buxifolia. Hybrids are usually less fertile than pure-bred species, and are occasionally quite sterile. Some hybrids, however, are as fertile as pure-bred plants. Hybrid plants may be again crossed, or even re-hybridized, so as to produce a progeny of very mixed parentage. This is the case with many of our roses, dahlias, begonias, pelargoniums, and
other long or widely cultivated garden plants.
