422 PLANTS, SEEDS, AND CURRENTS 



of Polygonum maritimum. The seeds of J uncus acutus, like those 

 of other species of the genus referred to on a previous page, would 

 probably become sticky when wet, and would thus adhere firmly 

 to a bird's plumage. 



The Efficacy of the Wind in the Oversea Dispersal of Seeds. — Much 

 has been written, but few actual facts have been recorded relating 

 to this subject. Mr. Wallace in his Darwinism (1889) made a 

 strenuous appeal for the paramount influence of winds over birds 

 in transporting small seeds like those of Sagina and Orchis over tracts 

 of ocean 1000 miles in width. " For each single seed carried away 

 by external attachment to the feet or feathers of a bird, countless 

 millions (he says) are probably carried away by violent winds ; and 

 the chance of conveyance to a great distance and in a definite direction 

 must be many times greater by the latter mode than by the former " 

 (p. 373). He based his opinion upon the careful comparison of the 

 size of a number of small seeds with those of quartz grains, 2^o^ n °^ 

 an inch across, found in deep-sea deposits 700 miles from land and 

 regarded by Sir John Murray as distributed by the winds. 



There seems to be no question about the fitness of cryptogamic 

 spores for dispersion by winds across broad tracts of sea. It is 

 concerning the seeds of flowering plants that doubts would be raised. 

 The great contrast in weight between the lightest of small seeds, as 

 in those of orchids, and the average weight of a mushroom spore 

 (orchids, 8000-15,000 seeds to a grain, Wallace and Kerner; mush- 

 room spores, probably some hundreds of thousands to a grain) at 

 once indicates problems of a very different nature. With regard 

 to species of flowering plants represented in the Azores, the following 

 measurements of size and weight were obtained by the author with 

 the exception of those for Sagina procumbens which are supplied by 

 Wallace in the work above named. 



Sagina procumbens, of an inch, 12 ^ 00 of a grain. 

 J uncus communis, 

 Erica azorica, 

 Calluna vulgaris, 

 Sibthorpia europaza, 

 Menziesia polifolia, 

 Thymus serpyllum, 

 Lysimachia nemorum, 

 Cotyledon umbilicus, 



i 



7 5 

 1 



55 

 1 



517 



1 

 5^0 



1 



7T0 

 1 



40 



>,A 



1 



T5 



1 



5500 



2T00 

 1 



2000 

 1 



Note. — The relation between size and weight varies with the form 

 of the seed. Thus the rounded seeds of Thymus are much heavier 

 for their size than the oblong somewhat flattened seeds of Calluna. 



Yet minute as the seeds of many widely distributed flowering 

 plants may be, Wallace gave no weight to a very important factor 

 in the continuous action of gravity, which seems to nullify any fitness 

 such seeds might appear to possess for transport by the winds across 

 a broad tract of ocean. All that follows, relating to this factor, 

 is based either directly or indirectly on materials supplied by Mr. 



