1522 A TEXTBOOK OF THEORETICAL BOTANY 



distribution in time rather than space. Mountain streams may also move 

 seeds mechanically without reference to their abihty to float, in the same 

 way that they move stones and rocks, by mere force. It is a common 

 experience to find alpine species growing by streams far below^ their usual 

 level on the mountains, having been carried down either as seeds or as 

 portions of plants. 



Enormous quantities of seeds and fruits are carried in the drift floating 

 on rivers, including many which are non-floating by themselves. From a 

 patch of drift in an American river, McAtee isolated 2,490 seeds, many 

 of them non-buoyant, as well as tubers of Cyperus. 



Millions of seeds must yearly be carried down to the sea by rivers and 

 the vast majority lost. It is therefore important for a fresh-watei plant that 

 it should not float for too long a time. Two or three days is usually sufficient 

 to allow adequate dispersal. 



For successful dispersal by sea a much longer period of buoyancy is 

 desirable and, furthermore, the seedlings must be resistant to salt water 

 as they will have to establish themselves on beaches or estuaries. Some 

 maritime plants get carried by the tide a long way inland on big estuaries 

 but they will always be within the reach of water which is at least brackish. 

 It follows that sea dispersal is practically confined to shore-living species. 

 The chief exceptions are those seeds which may be transported on floating 

 logs. They may come from inland forests, but the seeds must be imper- 

 meable to salt water if they are to survive the journey and the odds against 

 their successful establishment at the end of it are considerable. 



The number of species which are dispersed by the sea is not large and 

 they are mostly to be found in S.E. Asia and Polynesia, where the many 

 islands offer suitable stepping stones in dispersal. Ocean currents move 

 over vast distances, it is true, but many of them are of little value for 

 dispersal, inasmuch as they either lead to destinations where the climate is 

 completely different from that at their source, or else they touch no land 

 on their way, so that seeds die on the journey. If the salt water penetrates 

 a seed while it is still floating it either dies or it germinates. In either case 

 it is lost. A well-known example of the first difficulty is afforded by the 

 N. Atlantic drift, which carries seeds from tropical America and the West 

 Indies to the shores of N.W. Europe. Seeds of Entada, Mucuna, and 

 Ipomoea may survive this journey of at least a year's duration, but naturally 

 they are quite unable to grow on the cold shores where they come to land. 



Many sea-borne species must have commenced their wanderings during 

 the Tertiary period when the disposition of land and sea was very different 

 from that of the present day, so that their distribution can only be under- 

 stood on the basis of sea drift across regions which are now land. Thus 

 there are a number of such plants common to the shores on both coasts of 

 tropical America which probably passed across the isthmus of Panama 

 in the Pliocene period, when it was submerged. 



Among the routes of successful migration by sea are the following: 

 (i) Australia and Malaya to Polynesia; (2) Malaya across the Indian Ocean 



