Clare Island Survey — Phanerogamia. 10 71 



of the Hymenocyte fungus Amanitopsis vaginata, which are spherical and 

 smooth, and about one-third the diameter of Lycopodium spores. The 

 average terminal velocity of fall obtained was 6'07 mm. per second, which 

 is 46 per cent, greater than that deduced from Stokes's Law. The author 

 suggests that the excess over the calculated velocity may be due to surface 

 slip. Professor J. A. McClelland has suggested to me that the method of 

 calculating the density of the spores may be at fault. 



The effect of the great reduction in size of spores of Cryptogams is to 

 give them a very low rate of fall in comparison with the seeds of Flowering 

 Plants, and a consequent power of very wide dispersal by air-currents. Buller 1 

 finds the velocity of fall of the spores of Hymenomycetes to range from - 3 to 

 6'0 mm. (= -gij to J inch approximately) per second — a velocity only about Vo- 

 te -iiVo of that of a Dandelion seed, one of the more efficient of plume-seeds. 

 Further, these spores are in many Cryptogams produced in vast numbers. 

 A large example of the Giant Puff-ball (Zycoperdon bovista) is estimated to 

 discharge 7,000,000,000,000 spores, and a Common Mushroom (Psalliota 

 campestris) of 8 cm. diameter to contain 1,800,000,000. 2 So that, as was 

 said at the beginning, these spores are to be reckoned as a common constituent 

 of atmospheric dust ; and it is no exaggeration to say that they are probably 

 carried by the wind to the most remote corners of the globe. 



None of the seeds of the Flowering Plants attain such small dimensions as 

 the foregoing ; but nevertheless a number are exceedingly minute and light. 

 For instance, a seed of Hctbenaria conopsea weighs '000008 gram, and one of 

 Parnassia palustris -00003 gram. 3 Furthermore, " to enable these seeds to 

 float in the air for as long a time as possible, they are more or less flattened, 

 and their centre of gravity is so placed that they always present the broad 

 side to the direction of descent." But most of the minute seeds among our 

 native plants— for instance, those of the smaller Caryophyllaceae and 

 Cruciferae, the Poppies, &c. — are roundish in shape, and devoid of special 

 adaptations for wind dispersal ; they fall with comparatively high velocities, 

 as will be seen later. To pursue this line of thought, it may be remarked that 

 despite the advantage as regards dispersal afforded by small seeds, as 

 compared with large, these are not the rule in our flora. Clement Eeid 4 

 reckons 17"6 per cent, of the phanerogamic flora of the British Islands as 

 having " small seeds," and 24'5 as having " large seeds," the balance of 57'9 

 having seeds of intermediate size. The low percentage of small-seeded 

 species would seem to show that the size of seeds is determined by factors 

 other than ease of dispersal. 



1 A. H. R. Bullek : The Production, Liberation, and Dispersion of the Spores of Hymeno- 

 mycetes. Brit. Assoc. Report for 1909, pp. 675, 676. 1910. 



2 Boller : he. cit. 3 Kerner : Nat. Hist, of Plants, ii., p. 851. * Tern, cit., p. 21. 



