THE ANGIOSPERMAE 1523 



to E. Africa; (3) Western America to Polynesia; (4) South America to 

 Africa and thence to southern Asia; (5) Around the Arctic Sea. All but the 

 last of these routes are inter-tropical. 



Sea dispersal has been going on for such a long time that many littoral 

 plants have become virtually pan-tropical or pan-arctic and it is now a 

 matter of great difficulty to say whence they originated. Only in cases 

 where thev happen to have a number of close systematic relations in one 

 area can it be concluded that they probably belong to the area thus indicated. 



Of prime importance in deciding the success of sea migrations is the 

 qualitv of the land on which the seeds are eventually thrown. Plants of 

 estuarine muds, for example, will stand no chance of establishment on coral- 

 islands, nor will rock or sand plants be any more successful on a low-lying 

 and swampy coast. Great numbers of drifting seeds must be wasted for 

 these reasons, nevertheless a few species have reached wide distribution 

 by this means. Such littoral plants must generally have been the first 

 arrivals on new land, since before their establishment there would be little 

 inducement for birds, bringing other species, to visit the area. With few 

 exceptions the littoral species with wide distributions belong to large 

 genera with many inland species, but whether they were the progenitors 

 of the inland species or specialized derivatives from the latter is a matter 

 for speculation. 



A few living seedlings, like those of the " viviparous " Rhizophoraceae 

 which are dropped from the trees into the tidal mud of the mangrove 

 swamps, are floated away by sea currents and are dispersed like seeds, 

 except that they cannot survive as long as most seeds and are more liable 

 to be eaten or destroyed. Nevertheless Rhizophora mucronata ranges all 

 round the Indian Ocean and into Polynesia, evidence of considerable 

 success in flotation. 



The influence of ice as an agent of dispersal should not be overlooked. 

 Icebergs which break off from the foot of a land glacier often bear a load 

 of stones and soil, in which many seeds may be included, and the wide 

 dispersal of many Arctic plants must be largely due to this means of 

 transport. The ice may also drift southwards, especially in the Atlantic, 

 and Darwin considered that the large number of European plants occurring 

 in the Azores might well be due to ice transport during or after the glacial 

 period. 



There is a wealth of observations recorded touching the fascinating 

 subject of seed dispersal and those who wish to read further should consult 

 H. N. Ridley's " Dispersal of Plants Throughout the World " and H. R. 

 Guppy's " Studies on Seeds and Fruits " and " A Naturalist in the 

 Pacific", to which we ourselves are indebted for many of the foregoing 

 particulars. 



We have occasionally referred to the productivity of plants in respect 

 of seeds in certain special cases. Productivity of seeds is, however, only 

 one factor, it needs to be considered in the light of germinability and 

 capacity for establishment to give a truer picture of the reproductive 



