DISTRIBUTION OF SPHAGNUM AND CAREX 355 



to an altitude of fifteen or twenty miles. Although, therefore, this 

 would exclude all the flowering plants,even those with plumed seeds, 

 the matter assumes a different aspect in the case of the spores of 

 cryptogams. Small as it is, the orchid seed may fall fifty times as 

 fast through the air as the spore of a mushroom ; and an initial eleva- 

 tion of at most 3000 feet would be needed for a successful traverse 

 by a mushroom spore of 2000 miles of ocean before a wind blowing 

 with a force of fifty miles an hour. This is the maximum altitude ; 

 but the average elevation required, as indicated by Buller's falling 

 rates given in the table in Chap. XIX, would be only half this amount. 

 Though I have no direct data for the Peat-mosses it is not probable 

 that their spores would require an initial elevation exceeding the 

 average height of a lofty mountain-range like that of the Andes, up 

 the slopes of which the ascending currents of air would be able to 

 carry cryptogamic spores to a suitable starting-level several thousands 

 of feet above the sea. (If the spores of Polytrichum could serve us as 

 a guide, the falling rates of which are given in the same table, the 

 initial altitude requisite for Sphagnum would not far exceed 1000 

 feet.) The occurrence of these up-draughts on the sides of high 

 mountains is well known, and the matter is dealt with afterwards. 

 In this manner the spores of cryptogams would be brought within 

 the influence of the upper air-currents and distributed far and 

 wide. 



Summary 



1. On finding that the Sphagnum floras of the eastern and western 

 hemispheres become more and more differentiated as one recedes 

 from the north polar region, the author turned to the Carices and 

 received the same reply. It was at the same time discovered that 

 in their distribution both genera reproduce many of the problems 

 which the plant-world presents in the case of islands and in the 

 floras of the great land-masses of the southern hemisphere. It also 

 appeared from the behaviour of Sphagnum that the lower plants 

 require a larger area than the higher plants for evoking the full 

 effects of the differentiating process, and that in this respect our globe 

 may not be large enough for lowly organised plants (pp. 332-4). 



2. A table is given illustrating the effect of the divergence of the 

 land-masses from the north on the distribution of Sphagnum and 

 Carex. A comparison is then made of their behaviour in the North 

 American and Eurasian land-masses, and it is shown for both genera 

 that whilst the species common to both gather in the north, those 

 separately held congregate in the south. Thus it is indicated for 

 Sphagnum that 87 per cent, of the species common to both North 

 America and Eurasia are Arctic and Subarctic species. Of the 

 species separately held very few are found north of the temperate 

 zone. Thus in both the east and the west only 5 or 6 per cent, of 

 the species confined to the respective hemispheres occur in Arctic 

 or Subarctic latitudes. The Carices tell the same story in both 

 hemispheres, the proportion of species which North America holds 

 in common with Eurasia being 93 per cent, in the Arctic regions, 

 40 per cent, in the Subarctic regions, 24 per cent, in temperate lati- 



