1514 A TEXTBOOK OF THEORETICAL BOTANY 



mgm. have been collected in dust 600 miles from land. Seeds of low 

 specific gravity and of larger relative surface could probably travel as far, 

 even if they weighed ten times as much. 



The reason for the relative ineffectiveness of wind over the sea is 

 probably that the wind moves in waves rather than as a continuous current. 

 On land a seed or fruit may therefore be dropped and picked up again, 

 time after time, but this cannot happen so easily over water. 



Wind dispersal means, as a rule, wide dispersal. Ridley has pointed 

 out that an annual plant with a plumed seed {e.g., Senecio vulgaris) capable 

 of travelling 25 miles in a year, could girdle the earth in 1,000 years, an 

 insignificant fraction of the life-span of a species. 



Space will not allow more than a mention of a few types of seeds which 

 are wind-dispersed. Dust-seeds form the smallest and lightest group of 

 seeds. In addition to their minute size and light weight, they often have 

 the advantage of having a flattened, wing-like testa {Rhododendron spp.), 

 or one drawn out terminally into long extensions {Buddleia, Nepenthes) 

 which have somewhat the same effect as wings. 



The family of the Orchidaceae are the most notable for the possession 

 of dust-seeds, some weighing as little as 0-004 ^g"^-. which are pro- 

 duced in tens of thousands or even millions per plant. They are conveyed 

 by wind all over the world but the difficulties attending their establishment, 

 and the immense mortality, are vividly illustrated by Darwin's calculation 

 that if all the seeds of one plant of Orchis fnaciilata were to germinate and 

 grow to maturity, the great-grandchildren would suffice to cover the whole 

 land surface of the globe. 



Such masses of small seeds present a problem analogous to that of the 

 large mass of spores in the spore capsules of Hepaticae, in that they are 

 difficult to break up in the first place. The Hepaticae make use of elater 

 cells. A number of Orchids have achieved an analogous remedy in the 

 shape of long, hygroscopic hairs which grow from the placentae among 

 the seeds. These by their twisting movements separate the seeds from 

 one another, in a manner quite similar to the action of the elaters in 

 Hepaticae. 



Many seeds which are not furnished with any structural pecuHarity 

 aiding wind dispersal may nevertheless be blown for considerable distances 

 by reason of their very light weight. SaHsbury has made extensive measure- 

 ments of seed weights among plants of the British flora. In his published 

 tables he groups species according to the types of vegetation in which they 

 occur. Among 98 species of herbs from open, unshaded habitats, the 

 average weight, including both seeds and small fruits, was only 1-3 mgm.; 

 ranging from Sagina apetala, 00075 mgm. and Limosella aquatica, 0-009 

 mgm. to Elymus arenarius, 8-9 mgm. The average weight of the " seeds " 

 of thirteen species of common meadow grasses was only o-8 mgm. 

 Although a good deal bigger than the " dust " category of seeds, objects 

 as Hght as these would easily be carried about by strong winds. 



The subjoined table, after Salisbury and Fisher, shows how the mode 



