ii6 A NATURALIST IN THE PACIFIC chap. 



seeds of Arenaria (Honckeneya) peploides, but in a fashion quite 

 unique. The test is thin but impervious, and has no buoyancy ; 

 the curved embryo also sinks ; and the floating power arises 

 from the air contained in the loose spongy albumen, around which 

 the embryo is coiled (see figure). A more normal component 

 of the second group is represented in some Leguminous seeds, 

 perhaps of Lathyrus maritimus, that occur regularly amongst the 

 stranded seed-drift of the north coast of Devon. Here the kernel 

 of the seed is buoyant. The seeds of Euphorbia paralias are 

 indebted for their floating capacity to a layer of spongy tissue con- 

 taining large air-spaces placed between the kernel and the 

 chitinous outer test, neither of which possess any floating power 

 (see figure). They thus belong to the second section of the third 

 group. 



The fruits of Cakile maritima, Crithmum maritimum, Matricaria 

 inodora, and Scirpus maritimus, all belong to the first subdivision 

 of the third group where the air-bearing tissue exists in the 

 peripheral coverings, the seed or nucleus in all cases sinking. With 

 Cakile maritima there is a light spongy outer case of aeriferous 

 tissue, which, however, soon loses the epidermis, a circumstance 

 that probably explains the limited period of flotation of about a 

 week. The walls of the mericarp of Crithmum maritimum are 

 composed of spongy cellular air-bearing tissue with a persistent 

 epidermis, and the floating powers of the fruits are consequently 

 great. The achenes of Matricaria inodora have beneath the 

 epidermis a layer of buoyant tissue, and their structure is similar to 

 that found with the buoyant achenes of littoral species of Wedelia, 

 plants of the same order of Compositae that are found on the 

 Pacific islands. The cause of the floating power of the fruits of 

 Scirpus maritimus lies entirely, according to Kolpin Ravn, in the 

 air-bearing cells of the epidermis. The reader will find the results 

 of my experiments on the buoyancy of the seeds in Notes i6, 17, 

 and 18. 



Summary of the Chapter. 



(i) Following the main lines of Schimper's classification of 

 those of the Indo-Malayan region which possesses for the most 

 part the same species, the buoyant seeds and fruits of the littoral 

 plants of the Pacific islands are classed in three groups : the first 

 where the cavity of the seed or fruit is incompletely filled, the 

 floating power arising from the empty space ; the second where the 



