180 PLANTS, SEEDS, AND CURRENTS 



for an oversea traverse; yet even here the impervious outer skin 

 is deficient at the hilar or radicular end of the seed. 



A curious feature in the buoyant behaviour of the seeds of Anona 

 palustris is that they are, like other seeds of the genus, permeable to 

 water and air. When dry they behave hygroscopically and vary in 

 weight 2 or 3 per cent, according to the humidity of the surrounding 

 air. Fresh and moist from the fruit they owe their floating powers 

 entirely to the layer of cellular tissue lining the seed-shell, no other 

 part of the fresh seed possessing independent buoyancy. An exami- 

 nation of seeds that had been afloat some weeks showed that until 

 they became water-logged or saturated with water, there was no 

 soaking of water into the seed substance, but rather a vital process 

 of absorption of water, such as occurs in seeds imbedded in the 

 ripe fruit. Seeds that had been afloat for three weeks, after being 

 placed in a dry state in the water, were found to have taken up water 

 only to the extent that it occurs in seeds in the fruit, the interior 

 of the seed being fairly dry. Sufficient moisture had been absorbed 

 to restore the normal moist condition of the seed as it lies in the 

 fruit, and the embryo had resumed its original full outlines. There 

 were no signs of saturation. 



A point of importance is concerned with the degree of salinity 

 that the floating seeds of Anona palustris will withstand without 

 injury to their powers of germination. In the river Guayas the 

 germinating seeds were floating unharmed in brackish water that 

 at high water had a density of about 1-003. From the association 

 of this tree with the mangroves, the germinating fruits and seeds of 

 which are not affected by sea-water, it is probable that its seeds 

 can germinate in brackish water, though it is not likely that they 

 would do so in pure sea-water. Nor do I think that the Anona 

 trees would thrive in pure sea-water. The muddy banks of the Guayas 

 estuary, on which they flourish, are overflowed by brackish water 

 at high tide. But more observations on this point are needed. 

 These trees grow in brackish water in the Bahamas (Harshberger, 

 p. 691); and it is apparent that on the South Florida coasts, where 

 they are associated in the swamps with the mangroves, the water 

 must at least be brackish. But they are only halophilous by force 

 of circumstances. Their great development in the inland regions 

 of South Florida has taken place around fresh-water lakes, and 

 such conditions are evidently the most congenial for their growth. 



A singular point arises as to the practical bearing of these observa- 

 tions and experiments on the occurrence of Anona palustris on both 

 sides of the tropical Atlantic. It is just possible that the seeds 

 could safely traverse the Atlantic in the Main Equatorial Current 

 from the Gulf of Guinea to Brazil, a passage which would require 

 a period of two or three months, this being the only available trans- 

 atlantic route for seeds of their floating capacity. Yet Anona is 

 now almost exclusively an American genus, and if there has been 

 an interchange America has the first claim to be considered the 

 giver. But this would compel us to suppose either that the seeds 

 had been able to withstand the six to eight months' passage involved 

 in the Counter Equatorial Current, or that they could accomplish 



