THE AZORES 



423 



Lloyd Praeger in his botanical memoir published in the reports of 

 the Clare Island Survey in 1911 (Proc. Roy. Irish Acad.). The appli- 

 cation of his data to the Azorean flora is my own, and possibly the 

 author may pardon me for making such a free use of his work ; but 

 it will best express the measure of my indebtedness. 



The contrast in weight between the smallest seeds of flowering 

 plants and the spores of cryptogams, reflects the difference between 

 inefficient and efficient dispersal by winds over great distances. 

 Mere reduction in size, writes Lloyd Praeger (p. 79), is not carried 

 far enough in the flowering plants to produce efficient dispersal by 

 winds. With their seeds we can detect a certain amount of relation 

 in their responses to the action of gravity between their size and 

 falling rates ; but quite another order of things presents itself in the 

 case of cryptogamic spores. Small and light as it is, the seed of an 

 orchid falls through the air at least fifty times as fast as the spore 

 of an ordinary mushroom. Here I have taken the average terminal 

 velocity of an orchid seed at one foot per second and of a mushroom 

 spore at 6 mm. a second. It will be seen below that according 

 to the data given in Prof. Buller's British Association paper (1909, 

 p. 675) the average falling rate for hymenomycetous spores would 

 probably be much less; but in order not to overstate the contrast 

 the most rapid rate has been chosen. This comparison will serve 

 to illustrate the remark in Lloyd Praeger' s paper (p. 70) that the 

 behaviour of small particles falling in air differs from that of larger 

 bodies, inasmuch as with continual reduction in size the impelling 

 force of gravity becomes rapidly smaller in comparison with the 

 decrease of resistance offered by the air, so that very small velocities 

 result. 



At the close of Chapter XVI. this matter is briefly mentioned in 

 connection with the Peat-mosses. Here it is treated more at length 

 with reference to the Azorean flora. It would appear from the 

 numerous experiments of Lloyd Praeger on the falling rate of seeds, 

 using the term " seed " in a general sense as implying in the words 

 of this writer the unit of dispersal, that except in the case of plumed 

 seeds of the lightest weights, such as those of Typha and Epilobium, 

 we could not appeal to the winds for the transport of even the smallest 

 seeds of flowering plants, this agency being only available for the 

 spores of cryptogams. 



To postulate the effects of gravity, as indicated by the falling 

 rate of seeds and spores, an initial altitude at the starting-place must 

 be assumed. If the wind has been effective in stocking the Azores 

 with plants we must regard Southern Europe as the starting-place, 

 since that is the source of the great majority of the small- seeded 

 flowering plants. Taking the distance of this group from the nearest 

 coasts of Portugal at about 800 miles, I have below given the minimum 

 initial elevation that would be required for seeds and spores to reach 

 the Azores with a favourable wind blowing with the force of a strong 

 gale at fifty miles an hour. The falling rates of the spores are taken 

 from some of the results obtained by Prof. Zeleny and Mr. 

 McKeehan, as well as by Prof. Buffer, which are given in the 

 British Association Report for 1909 (pp. 408, 675) and in Nature 



