CH. XIV] WIND-DISTRIBUTION OF SEEDS. 181 



The fact that seeds are widely scattered is proved by 

 the plants which grow on the walls of ruined buildings, 

 or in the mould accumulating in the tops of pollard trees, 

 where the seeds had certainly not been sown by man. 

 So numerous are the plants growing in such places that 

 Floras, i.e. lists of the vegetation, have been compiled for 

 Cologne Cathedral (in its unfinished condition), the Colos- 

 seum at Rome, for certain church towers in France, and 

 for the pollard willows near Cambridge. 



The chief means by which seeds are scattered, are the 

 following : 



I. They may be blown by the wind. 



II. They may be carried in the form of burrs 

 adhering to the hair of animals. 



III. They may be swallowed by animals, and may 

 germinate after passing through their bodies. 



I. Wind- Distribution. 



The spores of Mucor and those of the fern supply 

 instances of reproductive units whose distribution is 

 facilitated by minuteness. The seeds of flowering plants 

 are not generally so small as to approximate to the dust- 

 like character of spores, but the seeds of some Orchids are 

 exceedingly minute and are doubtless far more readily 

 wafted by currents in the air than is possible in the case 

 of more massive seeds. The more common adaptation 

 to aerial carriage is a specialisation in the matter of form. 

 Many seeds have a thin membranous border which 

 increases their area without perceptibly increasing their 

 weight, so that when freed from the mother plant they 



