PHYSIOLOGY 



291 



can then spring directly from the Fern-leaf (APOSPORY, in varieties of 

 Athyrium and Aspidium). 



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 exterminate 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 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 dis- 

 charge from their receptacles ; by animals ; and also by their accidental 

 transportation by railroads and ships. 



To ensure the dispersal of seeds by the wind, all those contriv- 



Fig. 220. — Winged seed of Bignonia mucronata. (Nat. size.) 



ances 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 fruit-walls, 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 (Dipterocarpus) or of the ovary (Acer, Fraxinus, 

 Ulmus, Polygonum, Bobinia, Gleditschia, and the fruits of manj r Umbelli- 

 ferae), or of the seeds themselves, as in the winged seeds of the 

 Bignoniaceae (and many Temstroemiaceae). 



In a Bignonia seed (Fig. 220), with its widely outspread, glossy 



