REVIEWS AND NOTICES OF BOOKS. I 25 



Fagus sylvatica x Quercus pedunculata, Quercus pedunculata x 

 Juglans nigra, Quercus pedunculata x Fagus sylvatica, Quercus 

 pedunculata y. Juglans regia — have been attempted in 19 14. 

 Bi-generic hybrids are unlikely to be produced among trees, but 

 when one looks to the amazing results which have been obtained 

 in this way among orchids and a number of other plants, it would 

 be foolish to express any decided opinion until the experiments 

 have been carried further. 



One difficulty there is with these hybrids as subjects for 

 forestry. In the cases of hybrid willows and poplars, which are 

 usually propagated by cuttings, there is no difficulty in multi- 

 plying the individual plants to any desired extent, and even in 

 the case of elms layering as a method of vegetative reproduction 

 might be resorted to; but there are some kinds which cannot be 

 propagated so easily as these, in which budding or grafting must 

 be practised, and they are therefore useless to the forester. 

 Seed, even when it is produced, will not give an exact repro- 

 duction of the parent. 



The cause of the extraordinary vigour of first crosses is 

 obscure, and Professor Henry does not attempt an explanation 

 of it. He suggests it as possible that " the stimulus which 

 causes growth {i.e. cell-division) to commence and to continue is 

 some soluble chemical compound or enzyme," and that "tjie 

 enzyme in the hybrid may be more complex and more effective 

 than the enzymes in the species." 



What Professor Henry has to say about the association of the 

 vigour in the hybrid with the degree of relationship between the 

 parents is interesting. " Whether," he says, " the amount of 

 vigour in hybrids is directly associated with the degree of 

 relationship between the individuals which are crossed is a 

 disputable point, but one of practical interest in the selection of 

 parents for crossing experiments. One of my most vigorous 

 hybrids {Populus generosa) is derived from two parents so little 

 related that they are placed in two distinct sections of the genus. 

 A cross between two races of the common alder shows consider- 

 able vigour, though the parents are so closely allied that they 

 can only be distinguished by the most trivial characters. The 

 fact, if established, that different races when crossed give vigorous 

 progeny, should be turned to practical account in plantations 

 and forests where natural regeneration is looked for. The 

 introduction of a few lines of the Riga variety into a plantation 



