CROSS-FERTILIZATION IN PLANTS. As early as 1 793 the German botanist Sprengel discovered the main facts, with many details, of the rela tion of insect, to flowers. He observed the won derful adaptation of the structure of many flowers to this or that species of insect visiting them; he satisfactorily proved that many kinds of flowers are cross-fertilized by insects, and that this was the object, of the adaptations. He also observed that the presence of nectar and pollen attracted the insects and insured the •on tinuance of their visits. Several later writers showed that the cross-fertilization of plants was beneficial to them. These discoveries were greatly extended by Darwin, who perceived the value of this process io the life and mainte nance of the species. Darwin's obserTations were chiefly made on orchids, primulas. and so forth, and the subject was greatly extended by Hermann 1\ Hiller, who, after many years of research, published his results on flowers and the structure and wonderful adaptations of the insects which visit them, in his elaborate volume entitled The Ferri) int ion of Flowers. It was further demonstrated by Darwin that the cross fertilization of plants results in increasing the size, height. rapidity of growth. strength. and vigor of the plant. and in increasing its fertil it y. As stated by Wallace (Darwinism, p. 309), these results were produced immediately, and not after several generations of crosses. Darwin planted seeds from cross-fertilized and self fertilized plants on two sides of the same pot, which Were. exposed to exactly the same condi tions. The result was that in most cases the difference in size and vigor in the two set. was marked, while the plants arising fertilized parents also produced more and finer seeds. These experiments confirmed the experi ence of breeders of domestic animals, where the evil effects of breeding members,' of the same lit ter of dogs. or intermarriage between blood-rela tives, result in sterility and degeneration. The results of cross-fertilization between flowers of the same species, and the great interest excited IT Darwin in the subject, have led to a detailed study of the various contrivances by which flow ers secure crow-fertilization, and this has led to the view that at least the irregular flowers, such as those of the pea, sage, and the orchid families, are, in reality. due to the visits of in sects—a subject specially worked out by the Rev. George Nenslow in his 0 rigi n of Floral
Struct ures.
.Modes of Securing Truss - Pert iliza t ion in Pia f the simpler modes there are four— viz. (1 ) By dichogamy, where the anthers and the stigma become mature or ready for fertiliza tion at slightly different times on the same plant, as in the geranium. thyme, arum, and others; (2) by the flower being self-sterile with its own pollen, as in the crimson flax; (3) by the sta mens and anthers being so situated that the pol len cannot fall upon the stigma, while it does fall upon some bee or moth whieh carries it to the stigma of another flower (malva) ; (4) by the male and female flowers being on different plants, as in dimcious flowers, the pollen being carried to the stigMas by the wind or by insects. See POLLINATION 11L'AIPLEBEE.
The more complex mode by which exogamy is brought about is seen ( 1 ) in the ease of the loosestrife ( yt !Irani salicaria) . a polymorphous flower, in which the Siallle1IS and pistil differ in length and position, and the different stamens in the same flower have widely different degrees of fertility when applied to the same stigma; (2) in the case of the barberry, the irritable stamens of which spring up and dust pollen upon the insect that touches them; (3) in the case of others in which there are levers or processes by which the anthers are mechanically brought down the head or hack of an insect entering, the flower in such a position as to be carried to the stigma of the next flower it visits (Salvia, ]tenth) 1-t1 in plants which have slieky secre tion as (Polygala). which on the tongue or probo.:eis of an insect, and is borne to the stigma of another flower; *(5) in plants which have exploding anthers and other complex adjustments which thoroughly dust the insect, in the pea family. e.g. Medic:Ago falcata: (GI the entrapping of visitors in the spathes of Arum and Aristoloehia ; and (7) in the traps of the flower of .ks•lepias and Ph•sianthus. which catch flies, butterflies, and wasps by their also the complex arrangements of orchids, described by Darwin in his Fedi/I:010a of Orchids. It ap pears that insects are attracted to those flowers which are largest and most gayly colored. the smaller, more inconspicuous not receiving so ninny visitors. Bright-red flowers attract but terflies; blue flowers are especially attractive to bees, while dull-yellow or brownish flowers at tract flies.