XII CAUSES OF BUOYANCY OF SEEDS AND FRUITS 115 



cases such spaces are insignificant in size or absent altogether. 

 Speaking generally, however, there is, as Professor Schimper 

 observes, great similarity in the structure of the buoyant tissue in 

 the coverings of these fruits and seeds. The cell-walls are thin or 

 only slightly thickened, and detached air-bearing portions of the 

 tissue will float for many weeks. The great floating capacity 

 of these fruits and seeds is stated by this investigator to be 

 entirely due to the tenacity with which the air is retained in the 

 covering tissues. It is, however, noteworthy that in the case 

 of Scsevola Koenigii the fruits are just as well suited for dispersal 

 by frugivorous birds as by the currents, a significant circumstance 

 discussed in the next chapter. 



The second section contains those plants where the buoyant 

 tissue occurs inside the hard shell of the fruit or seed, such as 

 is found, for example, in Anona paludosa, Mucuna gigantea, 

 Hernandia peltata, Cycas circinalis, &c. Professor Schimper here 

 includes Calophyllum inophyllum and Ximenia americana ; but I 

 have before remarked that the buoyancy of their fruits is mainly 

 due to their buoyant kernels. This aeriferous tissue forms a layer 

 between the seed or nucleus and the hard outer shell. It is 

 described by the above-named authority as soft or friable and dark- 

 brown. The cells contain air and may be closely arranged or 

 separated by small interspaces, their walls being neither woody 

 nor suberous. 



The structure of the buoyant seeds and seedvessels of the littoral 

 plants of the British flora. 



The littoral plants with floating seeds or fruits form but a 

 section of the strand-plants of the British flora, scarcely a third, as 

 is pointed out in Chapter IV., of the total number. Though small 

 in number they exhibit great variety in structure ; and notwith- 

 standing that as far as they have been examined they may all be 

 referred to one or other of the groups and sections of the classifica- 

 tion adopted in the synopsis for the plants of the Pacific islands, 

 nearly every plant presents in the structure of its seeds or seed- 

 vessels a type of buoyant structure different from the others. 



The first group is represented by the seeds of Convolvulus 

 soldanella, which owe their floating power to the incomplete filling 

 of the seed-cavity. The second group, where the buoyancy arises 

 from the buoyancy of the kernel or nucleus, is illustrated by the 



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