142 NATURAL SCIENCE. ..p^.^. 



the commencement of a true prothallium. The production of 

 endosperm cells must coincide almost with the moment of im- 

 pregnation, which would clearly be rendered impossible by an 

 earlier production of cells, considering the path the male nucleus 

 must take. It may be that the pollen-tube when entering the 

 nucellus stimulates the young embryo-sac to assume the state 

 necessary for it to be fertilised later on, but there is a profound dif- 

 ference between mere accessory stimulation and fertilisation. Is it 

 possible to follow the little male nucleus, lost among the numerous 

 nuclei of the large embryo-sac, on its way to the oosphere, and to 

 determine the precise moment when it enters the bottom of the 

 oosphere ? The difficulties are all but insurmountable, and we at any 

 rate cannot but agree that Dr. Treub was justified in his determina- 

 tion to put before us the many interesting and unexpected facts he has 

 elucidated without waiting to settle this doubtless very difficult point. 



The Casuarineae are usually placed in that section of Apetalae 

 which includes also the Cupuliferae, Myricaceae, Juglandaceae, and 

 other families. It is in these that we should expect to trace affinity 

 with the Casuarinese, but in none is there any indication of the 

 curious structures and events just described, and Treub concludes 

 that the Casuarineae occupy a quite exceptional place among the 

 Angiosperms. 



For the great development of sporogenous tissue (recalling the 

 Cycads and Cupressus) and the large number of macrospores, the 

 greater number of which arrive at the formation of a sexual 

 apparatus, we must seek comparison with the Vascular Cryptogams. 

 The formation of a large number of prothallial nuclei in the fertile 

 macrospore before fertilisation (supposing Treub's view correct as to 

 the moment of impregnation), recalls what passes in a greater degree 

 in the macrospore of Gymnosperms, and indicates an intermediate 

 stage between the latter and the Angiosperms. The origin of the 

 sexual apparatus from a single cell, the similarity in aspect of the 

 "neighbouring cells" of the oosphere to " canal-cells," indicate an 

 homology with the archegonium of Gymnosperms and Vascular 

 Cryptogams, and that the " neighbouring cells " are not comparable 

 with the synergidae of Angiosperms. Finally, there is the most 

 anomalous mode of entry of the pollen-tube by way of the chalaza, 

 for which Treub suggests the following explanation. 



When Angiospermy originated, the micropyle lost its function of 

 conducting the pollen-grains which now germinated, not on the 

 nucellus as in the Gymnosperms, but at a distance, the pollen-tube 

 having to traverse the intervening space. There were two ways in 

 which the pollen- tube might reach the embryo-sac : the one used 

 before by the grain, through the micropyle, the other through the 

 chalaza. The genus Casuarlna is, as far as we know at present, the 

 only surviving example which chose the latter mode of entry, the rest 

 of the Angiosperms taking the former. 



