XII CAUSES OF BUOYANCY OF SEEDS AND FRUITS iii 



presented by several species of Leguminosae, as with Entada 

 scandens, some species of Mucuna, and Caesalpinia bonducella. 

 As with the Convolvulaceous seed, the embryo sinks and the seed- 

 shell has no buoyancy; but here the floating power is due to 

 the existence of a more or less symmetrical long central cavity 

 produced by the arching or bending outwards of the large 

 cotyledons which lie usually in close contact with the seed-shell. 

 This arching outward of the cotyledons depeids on a shrinking 

 process in the setting or final stage of the maturation of the 

 seed. The stages of the process may be traced in the immature 

 seeds, which are much larger and in some cases twice the size of 

 the mature seed. In this immature condition the seed-coats are 

 soft, and the flabby fleshy and thick cotyledons fill up the seed- 

 cavity. As the hardening and setting process continues, the 

 cotyledons diminish in size, become firmer, and gradually bend 

 outward, leaving a central cavity. This arching outwards is no 

 doubt in part the result of the contraction of the seed-tests during 

 the shrinking process. Considerable variation prevails in the 

 results, and where the cavity is very small the seed sinks. Further 

 details relating to this subject will be given in my treatment ot 

 some of the plants, and especially under Csesalpinia. But it may 

 be here remarked with reference to Hawaiian seeds of Mucuna 

 urens D.C., that although they are strictly referable to this group, 

 they display beneath the hard test, on the side beneath the raphe, a 

 scanty layer of dark spongy air-bearing tissue which is sufficiently 

 buoyant to float up detached portions of the test, but does not of 

 itself give buoyancy to the seed. The significance of this structure 

 will be subsequently pointed out. The seed owes its floating 

 power to the large central cavity, but this layer of spongy tissue 

 adds to its buoyancy. 



The section where the buoyancy of the fruit is connected with 

 unoccupied space in the fruit-cavity is extremely heterogeneous in 

 its composition. Every fruit has a method of its own, and the 

 great variety of causes of buoyancy of a mechanical character is 

 here exemplified. For instance, with Gyrocarpus jacquini and 

 Cassytha filiformis the cause of buoyancy is in the main the same 

 as that described in the case of the Convolvulaceae. The origin of 

 the floating power of the pods of Derris uHginosa is two-fold. In 

 the first place the seed or seeds but partly fill the pod, and in the 

 second place the seed is able to float of itself by reason of its 

 possessing, as in the seeds of Entada scandens, a large central 

 cavity produced by the arching out of the cotyledons during the 



