328 TRIGONOCARPALBS [CH. 



than the epidermis remains : there is a definite megaspore-mem- 

 brane surrounding the prothallus-tissue in which there were 

 probably two archegonia (fig. 494, A, a)^. The nucellus stands 

 free within the integument from which it is separated by a space 

 {s, fig. 494, A). Microspores are frequently met with in the pollen- 

 chamber in which they matured after their introduction through 

 the micropyle, probably with the aid of a drop of mucilage: 

 they are large oval bodies with an average size of 160/a x 100/x, 

 some reaching over 200/j. in length and, as Renault was the first 

 to point out, within a finely granulate exospore there are several 

 thin- walled cells : this statement was not accepted by some 

 authors but Prof. Oliver's researches have amply confirmed it, 

 and an examination of the original preparations convinced me 

 that Renault had correctly described the structural features. 

 Ohver shows that there are about 20 cells within each microspore 

 regularly arranged as rows of five wedge-shaped elements with 

 their pointed ends directed towards the centre, and he thinks 

 that these cells may have undergone further division to produce 

 sperm-mother-cells, though there is not such good evidence of 

 this as in the differently constituted microspores of Stephano- 

 spermum caryoides. No trace of pollen-tubes was found and it 

 is probable that the antherozoids were liberated by the rupture 

 of the dehcate prothallus-tissue. The tracheal sheath afforded 

 an adequate means of water-supply to the pollen-chamber and 

 this enabled the motile antherozoids to reach the archegonia. 



Stephanospermum caryoides Ohver^. 



This species, founded on a specimen from Grand'Croix, is a larger 

 seed (15-5 x 12-5 mm.) and more globular than S. akenioides ; there 

 is a shorter micropylar beak and a less developed peri-micropylar 

 trough. As in S. akenioides the nucellus is free from the base and 

 the two species conform to the same general type of construction. 

 The microspores are distinguished by the presence of only two in- 

 ternal cells which do not occupy the whole of the spore-cavity but 

 are surrounded by a large peripheral cell comparable with the tube- 

 cell in recent microspores, though there is no proof that a tube 



1 Brongniart (81) PI. xvi. figs. 4, 6. 



2 Ibid. p. 373, PI. xtn. figs. 17—20; xltv. figs. 36—43. 



