288 BOTANY 



fuse together in the act of sexual reproduction. The sexual cells of 

 Mosses and Ferns, apart from all other considerations, would not unite 

 because the spermatozoids of Mosses are attracted to the female 

 organs by sugar, while those of the Ferns are only stimulated by malic 

 acid. In the case of Phanerogams, a mixed union of sexual cells is 

 likewise prevented by various obstacles to pollination and fertilisation. 

 Occasionally, however, the sexual cells of different varieties, species, or 

 even genera have shown themselves able to unite and produce descend- 

 ants capable of development. Such a union is termed Hybridisation, 

 or bastard-formation, and its products hybrids or bastards. 



Through the demonstration of the possibility of hybridisation, the 

 sexuality of plants, for a long time doubted, was indisputably proven. 

 (With this object in view, hybrids were raised in great numbers by 

 Kolreuter as early as 1761.) It also demonstrated that the real 

 purpose of sexual union was the combination of the properties of both 

 parents, for transitional forms are found among hybrids which in many 

 characteristics resemble the male and in others the female ancestor, or 

 they may show an equal combination of the characters of both. 

 Less frequently it happens that the hybrid resembles one ancestor 

 almost exclusively. In such a case the attributes of the other ancestor 

 remain latent, and may appear quite unexpectedly, through atavism, 

 in later generations. Had one species simple leaves and the other 

 compound, their hybrid would have leaves more or less cleft ; or were 

 the flowers of one parent species red and those of the other yellow, the 

 hybrid frequently bore flowers with red and yellow markings (mosaic 

 hybrids), or which were orange-coloured. If an early blooming form 

 were crossed with a late bloomer, the hybrid would flower at a time 

 intermediate between the two. From these and similar differences 

 shown by hybrids, it became clear that the inherited characteristics of 

 both the male and female cells were transmitted by sexual reproduction, 

 and that the only function of the male fertilising substance was not, as 

 was at one time believed, merely to give an impetus to the development of 

 the egg-cell. A large number of spontaneous hybrids have been found 

 which have arisen naturally from plants with a special capacity for 

 hybridisation. That such natural hybrids do not oftener occur is due 

 to the lack of an opportune time or space for their development, and 

 also to the fact that in the case of pollination of flowers with different 

 kinds of pollen, that of their own species seems always more effectual 

 in effecting fertilisation. 



The more closely allied the parent plants, the more readily, as a rule, may 

 hybrids between them be produced. Many families seem to incline naturally to 

 hybridisation (Solanaceae, Caryophyllaceae, Jridaceae, etc.) ; others again develop 

 hybrids only occasionally or not at all {Cruciferae, Papilionaceae, Urticaceae, Convol- 

 vulacme, etc.). Even in the same family the related genera and species exhibit 

 great differences in the readiness with which they may be crossed. The Grape- 

 vine and also the Willow are easily crossed with other species of their own genus, 



