188 PLANTS, SEEDS, AND CURRENTS 



Coming to the mode of dispersal of this genus by the currents 

 it may be at once observed that it is the upper article or joint of the 

 fruit that is alone concerned. It is deciduous and readily detached 

 from the dry fruit, whilst the lower joint remains firmly fixed to the 

 stalk on the plant. The buoyancy arises from the development 

 of spongy air-bearing tissue in the upper article, which is very light 

 when dry and buoys up the enclosed seed which itself sinks in sea- 

 water. As shown in the subjoined table, the floating capacity will 

 usually allow the seeds to be carried between 150 and 200 miles. 

 Flotation in sea- water, as indicated by my experiments on the fruits 

 of C. lanceolata and C. maritima does not affect the germinative 

 capacity of the seed, nor in fact does prolonged immersion after the 

 fruits have sunk affect it. Martins found that germination took 

 place after fruits of C. maritima had been in sea- water for forty-five 

 days, though it is more than doubtful, as is brought out in Note 4 

 of the Appendix, whether they floated for more than a fraction of 

 this period. In my experiments germination occurred with fruits 

 of C. lanceolata that after floating for a week had lain for several 

 days at the bottom of the vessel. The lesson of the experiment 

 of Martins is that a submersion of forty-five days in sea- water will 

 not destroy the germinative capacity of the seeds of Cakile. 



The restricted floating capacity arises from a weak point in the 

 buoyancy equipment. The suture between the lateral halves of 

 the joint is exposed in the face of the articulation at its base; and 

 it is here that the water ultimately penetrates into the interior and 

 deprives the article of its floating power. In old weathered fruits 

 this suture becomes more or less patent in the face of the articulation 

 and there is a slit-like opening. From this cause the buoyancy of 

 the joints is not always constant, as is shown in the different behaviour 

 of those of Cakile lanceolata obtained from Jamaica and the Turks 

 Islands. 



Nevertheless, although the buoyancy as a rule is limited, there is 

 abundant evidence in the beach-drift of different regions that the 

 currents are effective agents in dispersing the fruits over limited 

 tracts of sea. I found the joints of Cakile to be characteristic con- 

 stituents of the smaller beach-drift in the south-west of England 

 (Cornwall and Devon), in Jamaica, and in the Turks Islands. Ser- 

 nander also alludes to them as occurring plentifully amongst the 

 Scandinavian beach-drift (pp. 122, 156, etc.). In all these localities 

 several of the stranded fruits were observed to be in a germinating 

 condition, and not infrequently seedlings were to be noticed growing 

 among the stranded drift- materials. 



One may here direct attention to the very interesting suggestion 

 made by Dr. Millspaugh that the differentiation of the species of 

 Cakile is a response to the requirements of dispersal by currents. 

 "The evolution for floatage" (he writes) "seems to have reached 

 its height in the new species growing upon the Alacran Shoals " 

 (C. alacranensis of Millspaugh). Here there is a great development 

 of spongy tissue in the upper joint ; and it is argued that " the 

 species-generating force in the genus seems to have been the develop- 

 ment of the fruit for disseminating the plants." Prolonged investiga- 



