318 BOTANY PART I 



especially in those arising from the hybridisation of different varieties 

 of the same species. 



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. 



It is doubtful if hybrid forms can be produced (graft-hybrids) 

 by a vegetative union of portions of two different plants (grafting, 

 budding) ; Laburnum Adami (Fig. 247), and more recently an inter- 

 mediate form between Mespilus germanica and Crataegus monogyna ( 12 ), 

 appear to be of this nature. As yet, however, attempts to experi- 

 mentally obtain these forms by vegetative or sexual methods have 

 not succeeded. (Cf. p. 252.) 



The Dissemination and Germination of Seeds 



If the seeds after their separation from the parent plant simply 

 fell upon the earth, the young seedlings would be injuriously restricted 

 to the place already occupied by the parent plant, and would also 

 spring up in such large numbers that they would mutually ex- 

 terminate each other. The dissemination of the seeds thus becomes 

 a necessity, and although a larger or smaller proportion perish in the 

 process, a small number eventually find themselves in a favourable 

 environment. ! 



For their DISSEMINATION, seeds (and also spores and other repro- 

 ductive bodies) make use of the same agencies as are employed for 

 the conveyance of pollen. Thus their dispersion is effected by means 

 of currents of air and water, by their forcible discharge from their 

 receptacles and by animals ; to these means must now be added 

 human intercourse as a by no means unimportant mode of dispersal. 



To ensure the dispersal of seeds by the wind, all those contrivances are of use 

 which serve to increase their superficial area with but small augmentation of their 

 weight. Of this nature are the hairy appendages of seeds and fruits, as in 

 Gossypium, Epilobium, Populus, Salix, Typha, Clematis, and the fruits of the 

 Compositae with their pappus, of Valeriana, etc. Compared with the accelerated 

 fall in a vacuum, the retardation exerted by the resistance of the air (by which the 

 opportunity for dispersal through the agency of the wind is enhanced) in the case 

 of Cynaria Scolymus is, in the first second, as six to one. Similar adaptations 

 for utilising the agency of the wind as a means of dispersal are the wing-like 

 appendages formed from the expansion of the sepals (Diptcroearjms) or of the 

 ovary (Acer, Frazinus, Ulmus, Polygonum, Robinia, Gleditschia, and the fruits of 

 many Umbelliferae), or of the seeds themselves, as in the winged seeds of the 



