PERMEABILITY AND CLASSIFICATION 97 



honducella^ that have long been known to be dispersed by 

 ocean-currents. Putting aside the question of adaptation to 

 modes of distribution, a view which I hold ought either to be 

 universally applied or to be discarded altogether, the strand 

 of a coral island would be deprived of several of its most 

 conspicuous and typical plants, such as Canavalia obtusifolia, 

 Colubrina asiaiica, Erythrina indica, Guilandina bonducella, 

 Ipomcsa pes-capra, Morinda citrifolia, Mucuna (species), Sophora 

 tomentosa, etc., if their seeds did not possess impermeable coats, 

 in most if not in all cases associated with the capacity of 

 prolonged vitality. 



Our views of the adaptive relations of impermeability, Iiiii>erme- 

 if it is valid to select cases in a world that is one mass of tdapte^n. 

 adaptation, might be a good deal coloured by the station 

 of the plants investigated. Thus, it might be considered 

 from a study of the plants of tropical coasts that imper- 

 meability was most characteristic of the seeds of beach and 

 estuarine plants dispersed by currents. But it proves to have 

 no exclusive association either with buoyancy or with a littoral 

 station. Thus quite a third of the seeds named in my 

 impermeable group {Adenanthera pavonina^ Guilandina bonduc, 

 G. melanosperma^ Ipomoea dissecta, Leucana glauca, and XJlex 

 eurapaus) have no floating powers ; and it may be safely said 

 that the great majority of the 180 " macrobiotic " impermeable 

 species of seeds named in Professor Ewart's list have no 

 buoyancy and no littoral station. In framing any views in 

 support of the adaptive relations of impermeability in seeds, 

 it would be wise to cast one's net widely and to adopt the 

 standpoint taken by Professor Ewart, that there is here a 

 general adaptation to soil-conditions, without committing 

 oneself to any mode of genesis of this quality. 



I will now deal with some plants of the variable group The lesson 

 possessing the two kinds of seeds, sometimes differing from ^oss^sing 

 each other in external characters as well as in the degree ^jg j^^ 

 of impermeability. They will serve to throw some light permeable 

 on the nature and relations of this quality. Reference has 



7 



