416 



GEAFT-HYBRIDS. 



Chap. XI 



whole case has been attributed by some auth< rs to bud- variation ; 

 but considering the wide difference between 0. laburnum and 

 purpureus, both of which are natural species, and considering the 

 sterility of the intermediate form, this view may be summarily 

 rejected. We shall presently see that, with hybrid plants, two 

 embryos differing in their characters may be developed within the 

 same seed and cohere; and it has been supposed that G. adami 

 thus originated. Many botanists maintain that G. adami is a 

 hybrid, produced in the common way by seed, and that it has 

 reverted by buds to its two parent-forms. Negative results are not 

 of much value ; but Beisseck, Caspary, and myself, tried in vain to 

 cross G. laburnum and purpureas ; when I fertilised the former with 

 pollen of the latter, I had the nearest approach to success, for pods 

 were formed, but in sixteen days after the withering of the flowers, 

 they fell off. Nevertheless, the belief that G. adami is a spon- 

 taneously produced hybrid between these two species is supported 

 by the fact that such hybrids have arisen in this genus. "In a bed 

 of seedlings from C. elongatus, which grew near to G. purpureas, and 

 was probably fertilised by it through the agency of insects (for 

 these, as I know by experiment, play an important part in the fer- 

 tilisation of the laburnum), the sterile hybrid G. purpureo-elongatus 

 appeared. 96 Thus, also, "Waterer's laburnum, the G. alpine-labur- 

 num, 97 spontaneously appeared, as I am informed by Mr. Waterer, 

 in a bed of seedlings. 



On the other hand, we have a clear and distinct account given to 

 Poiteau, 98 by M. Adam, who raised the plant, showing that G. adami 

 is not an ordinary hybrid; but is what may be called a graft-hybrid, 

 that is, one produced from the united cellular tissue of two distinct 

 species. M. Adam inserted in the usual manner a shield of the 

 bark of G. purpureus into a stock of C. laburnum ; and the bud lay 

 dormant, as often happens, for a year ; the shield then produced 

 many buds and shoots, one of which grew more upright at id 

 vigorous with larger leaves than the shoots of G. purpureus, and 



96 Braun, in ' Bot. Mem. Ray. Soc.,' 

 1853, p. xxiii. 



97 This hybrid has never been de- 

 scribed. It is exactly intermediate in 

 foliage, time of flowering, dark stria? 

 at the base of the standard petal, 

 hairiness of the ovarium, and in 

 almost every other character, be- 

 tween C. la'mrnum and alpinus ; but 

 it approaches the former species more 

 nearly in colour, and exceeds it in 

 the length of the racemes. We have 

 before seen that 203 per cent, of its 

 pollen-grains ai-e ill-formed and 

 worthless. My plant, though grow- 

 ing not above thirty or forty yards 



from both parent-species, during some 

 seasons yielded no good seeds ; but in 

 1866 it was unusually fertile, and its 

 long racemes produced from one to 

 occasionally even four pods. Many 

 of the pods contained no good seeds, 

 but generally they contained a single 

 apparently good seed, sometimes two, 

 and in one case three seeds. Some of 

 these seeds germinated, and I raised 

 two trees from them ; one resembles 

 the present form; the other has a 

 remarkable dwarf character with 

 small leaves, but has not yet flowered. 

 98 ' Annales de la Soc. de l'Hort. de 

 Parjs,' torn, vii,, 1830, p. 93, 



