4] DISPERSAL AND MIGRATION 125 



part of the capsule when it has dehisced and attained a certain degree 

 of desiccation, while the movements of teeth of Mosses are hygro- 

 scopic, curving backwards to open the capsule and disseminate the 

 spores. These actions take place chiefly in dry weather when 

 conditions are best for dispersal. Also able to move as a result of 

 hygroscopic changes are many awned fruits, etc. Finally, it should 

 be recalled that the spores or even the whole bodies of many of the 

 lower cryptogams are actively mobile, swimming by means of flagella 

 being particularly common among them. 



Barriers 



When we reflect that in many species of flowering plants, such as 

 Flixweed [Sisymbrium sophia) and Pigweed [Amaranthus retroflexus)} 

 a single individual may produce a million or more seeds in one 

 summer, and that some cryptogams, such as the Giant Puffball 

 [Lycoperdon (Calvatia) giganteum), may produce as many as several 

 million million spores, and yet none overruns the world,- it is obvious 

 that only an infinitesimally small proportion of the plant disseminules 

 produced ever attain their real biological raison d'etre. To realize 

 its full potentiality, a propagule must develop into an adult which 

 in turn reproduces. This stupendous mortality is due to the action 

 of various types of barriers — either to dispersal or to actual survival 

 — of some of which we have already seen examples as applied to 

 particular agents of dispersal. They are of four main types : 



(i) Physiographic, due to features of the earth's surface. The 

 most obvious of these for terrestrial plants are expanses of water, and, 

 for aquatic plants, bodies of land. Another physiographic barrier 

 is aff^orded by mountains — both directly by constituting a mechanical 

 impediment, and indirectly by changing climatic and allied conditions 

 such as air temperatures and currents. Many local winds are 

 caused by a combination of phvsiographic and climatic factors, and 

 constitute virtual barriers to dispersal in one direction even as they 

 may aid it in another. 



^ An individual of this annual species has been known to produce an estimated 

 2,350,000 seeds. 



^ It is said that an individual Giant Puffball can produce 7,000,000,000,000 

 spores, and it was calculated by the late Professor A. H. R. Buller {Researches 

 on Fungi, vol. Ill, 1924) ' that, if every spore of this puff-ball had germinated 

 and given rise to a puff-ball like its parent, and if every spore of the second- 

 generation puff-balls had likewise germinated and given rise to a puff-ball like its 

 parent, then, at the end of these two filial generations only, there would have 

 come into existence a mass of puff-ball matter equal to 800 globes the size of 

 the planet on which we live ! ' 



