,8^. A NEW GROUP OF FLOWERING PLANTS. 141 



that up to this stage the sexual apparatus remains unchanged. 

 When this stage is reached, the embryo-sac contains a great number 

 of nuclei, while in the dense protoplasm at the summit, endosperm 

 cells begin to be formed round them, and this formation descends 

 gradually to the base till the sac is filled with solid endosperm. 

 Shortly after the appearance of the first cells, development of the 

 embryo commences, by division of what must accordingly be now 

 the oospore. The embryogeny conforms to the ordinary dicotyle- 

 donous type, and Treub saw nothing particular to characterise it. 



Treub cannot indicate the precise moment of impregnation, not 

 having seen the male nucleus penetrate to the oosphere. The 

 otherwise universal test of the presence of a cell-wall will not apply, 

 for the oosphere of Casuarina has a cellulose membrane from its birth, 

 at a time when the pollen-tube has not even entered the nucellus. 

 Another indirect test fails. In Angiosperms carefully studied for this 

 purpose, the synergidae have generally quite a different appearance 

 before and after fertilisation, and often the fertilised oosphere elongates 

 considerably before the first division proves that it has been fertilised. 

 It is necessary from the position of the pollen-tube that the male 

 nucleus must (i) traverse the membranes of the pollen-tube and 

 embryo-sac, and enter the cavity of the latter ; (2) traverse a smaller 

 or greater part of the cavity ; (3) penetrate the oosphere from 

 below. Treub thinks fertilisation takes place much later than the 

 time of junction of the pollen-tube and embryo-sac, in fact, the moment 

 before the first endosperm cells appear in the top of the adult sac, 

 and adduces in support the following facts. The pollen-tube remains 

 attached to the embryo-sac ; we might expect if fertilisation had taken 

 place at once that the tube would have perished. There is, moreover, 

 a difference in the contents of the tube while the embryo-sac is still 

 growing, and when it is nearly or quite adult. In the latter, the 

 protoplasm has lost its former distinct appearance, and no trace of a 

 nucleus can be seen. The Casuarineae being tropical and sub-tropical 

 plants experiencing no intervening inclement season, there is no 

 reason for an interval between fertilisation and division of the 

 oospore, such as occurs, for instance, in the Autumn Crocus, where 

 fertilisation takes place in the autumn, but the oospore remains 

 undivided through the winter till the following May. Treub avers, 

 moreover, to have several times seen in adult embryo-sacs in which, 

 according to his view, fertilisation was on the point of taking place, 

 or even had just occurred, a spot in the base of the oosphere where the 

 mem.brane was very thin and sometimes drawn out to a point which 

 looked as if it were open. He also cites a case where an embryo-sac 

 had undergone considerable development before the pollen-tube had 

 got even close to it. 



This view implies that the production of the numerous nuclei in 

 the sac is not a consequence of impregnation as in other Angio- 

 sperms, but as independent of it as in Gymnosperms. It is, in fact. 



