290 BOTANY pakt i 



MORE LUXURIANT GROWTH. The fertility is often so enfeebled that 

 the hybrids either do not flower {lihododendron, Epilohinm), or are 

 sterile and do not reproduce themselves sexually. This enfeeblement 

 of the sexuality increases the more remote is the relationship of the 

 ancestral forms. Other hybrids such as those of Salix and Hieramim 

 remain fertile. 



The tendency to variability is often greatly enhanced in h3'brids, 

 especially in those arising from the hybridisation of different varieties 

 of the same species (cf. p. 289). 



Hybrids, particularly those from nearly related parents, frequently 

 produce more vigorous vegetative organs, they bloom earlier, longer, 

 and more profusely than the uncrossed plants, while at the same time 

 the flowers ai'e larger, more brilliant, and exhibit a tendency to 

 become double. The luxuriance of growth and the increased 

 tendency to produce varieties displayed by the hybrids have made 

 the whole subject of hybridisation one of great practical as well as 

 theoretical importance. 



As "Winkler has shown, intermediate forms can also be produced 

 under favourable circumstances by vegetative union (grafting). The 

 graft-hybrids (*") lietween the Nightshade and the Tomato arti- 

 ficially produced by Winkler leave hardly any doubt that certain 

 intermediate forms of uncertain origin in cultivation are of similar 

 nature. Thus Oi/tisus Adami (Fig. 221) is an intermediate form 

 between Cytisus laburnum and C. jjurpiireus. Another example is 

 afforded by the intermediate forms between Mcspihis germanica and 

 Crataegus monogyna, known as Bronveaux hybrids. These plants are 

 peculiar in often reproducing the 2:)arent forms l)y Inid- variation (Fig. 

 221). The actual conditions under which graft-hybrids arise are not 

 fully known. It is, however, at least doubtful Avhether a fusion of 

 two cells gives the starting point for the hybrid form. 



VII. Variation and Mutation {^) 



The laws which govern hybridisation must also hold for the fertilisa- 

 tion between two individuals of the same species when these exhilnt 

 differences in their hereditary characters. In this way descendants 

 are produced, characterised by a combination of jjaternal and maternal 

 characters ; fertilisation thus gives rise to new forms. These, it is 

 true, will not persist if their properties are not useful, or if on crossing 

 with other individuals of the species their characters do not prevail. 



The differences between the individuals of a pure species and the 

 individuals obtained by in -breeding from a plant need not, when 

 accurately observed, be so slight as is commoidy su])pose(l. The 

 deviations from the tj^pe which are ol)served can be placed in two 

 categories; we distinguish the fluctuating variations from mutations. 



c; 



