140 NATURAL SCIENCE. ^pr.l. 



in size without elongating, and often containing two fairly large 

 nuclei. In the fertile ovules, those macrospores which are well 

 developed, that is, the majority, include at the apex two or three 

 cells, rarely only one. These are usually naked, but sometimes 

 have cellulose walls, and their arrangement seems to indicate that 

 they have come from one primary cell. This is the sexual apparatus. 

 In the great majority of cases there is only one macrospore in the 

 nucellus in which the sexual apparatus has the cellulose walls ; this 

 is the future fertile embryo-sac. Occasionally, however, there is 

 more than one in which this is the case. The perfect equivalence of 

 the future embryo-sac and the sterile macrospores thus admits of no 

 doubt. 



Section 3 deals with the pollen-tube and the fertile embryo-sac. 

 Treub finds that the pollen-tube does not enter the ovary cavity, and 

 does not pass down the micropyle, but enters the nucellus at the 

 opposite end, namely, at the chalaza. Only one pollen-tube enters 

 each ovary. It passes down the style, through the tissue bordering 

 the nucellus, to the end of the vascular bundle which reaches the 

 chalaza. Here it branches, one or two short recurved limbs passing 

 outwards towards the surface of the ovule, while the main portion enters 

 the chalaza by the tail of a sterile macrospore, and passing rapidly 

 up leaves it again to approach and become fixed to the embryo-sac. 

 Sooner or later the pollen-tube contracts at a point in the middle of 

 the nucellus, and the upper portion becomes closed off, and separated 

 from the rest. This is evidently due to the fact that the embryo-sac 

 and nucellus grow considerably after the attachment of the pollen- 

 tube to the former, and this causes the rupture. 



The oosphere is distinguished from the one or two "neighbouring 

 cells " produced at the same time by its thicker membrane. In some 

 cases it is alone. Several examples have been noted in other Angio- 

 sperms of synergidae clothed with a cellulose membrane, but there is 

 not a single well-authorised case of an oosphere invested with a cellu- 

 lose wall before fertilisation. The appearance of the cell-wall is the 

 result of fertilisation, and is always regarded as an indication of its 

 accomplishment. Treub never saw antipodal cells in Casuavina. 



Another curious point is that the pollen-tube never joins the 

 macrospore membrane above the insertion of the sexual apparatus as 

 in other Angiosperms, but always at some point more or less distant, 

 sometimes diametrically opposite. 



The embryo-sac continues to increase in size, the number of 

 nuclei in the cavity also increases, while the sexual apparatus still 

 remains in the same state, the oosphere keeping the same aspect and 

 dimensions. The pollen-tube never enters the embryo-sac, but the 

 end applied to the wall contains a very distinct protoplasm, and, 

 probably, nuclei. 



If we consider as adult the embryo-sac which has reached the 

 dimensions it will keep till the appearance of the embryo, we find 



