XXVII LEMNACE^ 



407 



fruits of plants growing in the mangrove swamps, that when placed 

 in sea- water 40 per cent, sank during the first fortnight, whilst 15 

 per cent, floated after five or six weeks, but all were at the bottom 

 in two months. On the other hand, fruits from plants of the 

 swamps of the inland plateaux displayed much feebler floating 

 power, in some cases sinking at once, in others floating for a few 

 days, and in others again floating for a week or two. In this case 

 the outer cork-like covering proved to have lost most of its floating 

 power. 



From the number of empty seed-vessels found, both in the 

 floating and stranded drift, it appeared evident that the seed had 

 often rotted away during the flotation. It is apparent from these 

 observations and experiments that Scirpodendron costatum is not 

 suited for dispersal by currents over wide tracts of ocean. The 

 fruits might be able to float unharmed for a few weeks, but they 

 would be unable to accomplish much more than the 500 or 600 

 miles intervening between Fiji and the nearest groups to the west. 



LemnacecB 



This order, judging from the writings of Hegelmaier, Schenck, 

 and Hemsley, is represented by one or other of the common 

 species, Lemna minor, L. gibba, L. polyrrhiza, in various Atlantic 

 islands, as in the Bermudas, the Azores, Madeira, the Canary 

 Islands, and St. Helena ; but doubts frequently arise as to their 

 being truly indigenous. Lemna trisulca is regarded by Hemsley 

 as indigenous in the Bermudas. Lemna minor has been introduced 

 in recent years into Hawaii, where I observed it flowering and 

 sometimes fruiting abundantly in the heated waters of the ponds. 

 Two species found in other regions were recorded by Seemann 

 from Fiji, and I have come upon few other records of the occur- 

 rence of the order in the tropical islands of the open Pacific. I am 

 inclined to the opinion, based not only on the facts of distribution, 

 but also on the results of numerous experiments on the means of 

 dispersal, that this order has in most cases reached oceanic islands 

 with man's assistance. 



Some years ago I made a systematic study of the habits of the 

 British Lemnae, most of the results being published in the Linnean 

 Society's Journal (vols, xxix and xxx), as far as concerned 

 Lemna minor, L. gibba, and L. polyrrhiza. During this inquiry I 

 ascertained that with these species, as well as with L. trisulca, the 

 chances of a bird's carrying their fronds uninjured in its plumage 



