DIV. ii PHYSIOLOGY 319 



Hybrids are often recognisable by having the characters of inter- 

 mediate forms between the two parents. They may either be truly 

 intermediate, e.g. Nicotiana rustica $ x Nic. paniculata $ and Sorbus 

 aria x S. aucuparia (Fig. 267), or may in some characters resemble more 

 closely the male parent and in others the female parent. In exceptional 

 cases a hybrid may, even to minute characters, resemble the male 

 parent (some hybrids of the Strawberry) or the female parent. In 

 the great majority of cases it is all the same which plant is taken as 

 the male and which as the female parent. In some cases, however, 

 the hybrid A ? x B $ is clearly different from A $ x B 9 . 



The mingling of characters is often complete. Had one species simple and 

 the other compound leaves, their hybrid would have leaves more or less cleft 

 (Fig. 267) ; or were, the flowers of one parent species red and those of the other 

 yellow, the hybrid frequently bore flowers 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. Another type of hybrid which is less 

 commonly met with is that of the MOSAIC HYBRIDS. In this parts with maternal 

 characters are mingled with others which have the characters of the male parent. 



Xew characters appear in hybrids such as diminished fertility, a 

 greater tendency to the formation of varieties, and frequently a more 

 luxuriant growth. 



The fertility is often so enfeebled that the hybrids either do not flower 

 (Rhododendron, Epilobium), 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 

 Hieracium remain fertile. 



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 are 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. 



Inheritance in Hybrids ( 82 ). By the experimental study 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 is now the 

 problems of ' inheritance connected with hybridisation that are the 

 main centres of interest. For the study of heredity, however, 

 hybrids between species are far too complicated. It was by using 

 closely related forms that GREGOR MENDEL at Briinn discovered in 

 1866 certain laws, which, however, did not attract attention or 

 influence the progress of investigation till after 1900. At this 

 date they were re-discovered simultaneously by DE VRIES, CORRENS, 

 and TSCHERMAK. In order to obtain these laws or rules MENDEL 

 required to follow the behaviour of the hybrids through a number of 



