xii CAUSES OF BUOYANCY OF SEEDS AND FRUITS 109 



S. Scyphiphora hydrophyllacea. 

 S. Wollastonia glabrata. 

 G. Hippomane mancinella. 



Note. Here belong a species of Vitex, probably V. agnus castus, 

 the fruits of which occur in the stranded drift of the Sicilian 

 beaches, and also the British littoral shore-plants, Cakile maritima, 

 Crithmum maritimum, Matricaria inodora, and Scirpus maritimus. 



SECTION II. The buoyant tissue forms a layer inside the 

 hard test of a seed or inside the shell of he " stone " 

 of a drupaceous fruit, and to this cause the floating 

 power is mainly or entirely due. 



G. Mucuna gigantea (seed). 



S. Hernandia peltata. 



S. Excaecaria agallocha. 



S. Cycas circinalis. 



S. Pandanus odoratissimus. 



G. Anona paludosa (seed) of tropical America. 



Note. I have followed Schimper in respect to Pandanus, but 

 it might be by some placed in the first section of this group. 



Here belongs Euphorbia paralias, a British littoral plant, the 

 buoyant seeds of which occur in the stranded seed-drift of English 

 and Mediterranean beaches. 



In the following general discussion of the groups, reference will 

 be made only to the plants best illustrating the different varieties 

 of structure connected with buoyancy ; whilst mention of the other 

 plants will in some cases be found in other parts of this volume, as 

 shown in the Index ; and the matter is discussed at some length in 

 not a few of the species. 



THE FIRST GROUP. 



Of the first group, where the floating power is due to the un- 

 occupied space in the cavity of the seed or fruit, the Convolvu- 

 laceae offer the most typical examples. Here as a rule the 

 crumpled embryo fills the seed-cavity more or less incompletely ; 

 and it is on the relative size of the unoccupied space that the 

 sinking or floating of the seed depends. In those plants where the 

 seed sinks the seed-cavity may be almost filled, as in Ipomea 

 tuberculata, or densely packed, as in Ipomea pentaphylla, and 



