138 NATURAL SCIENCE. apr.l. 



embryo-sac, growing enormously, and crowding and absorbing its 

 sister-cells. In rare cases, one of the latter develops further, but 

 never far enough to give it the character of a sterile macrospore, a 

 rank which it certainly holds theoretically. Sometimes the nucellus 

 contains two or even several embryo-sac mother-cells. Thus, 

 Strasburger saw four in Rosa livida, and this supports the view that 

 we are dealing with rudimentary sporogenous tissue. In this case, 

 usually one of the daughter-cells from the commencement crowds 

 and absorbs, before they have reached any development, not only 

 its sister-cells, but also those arising by division of the other 

 mother-cells. Occasionally, some may develop enough to assume 

 the appearance of sterile macrospores, but it is very rare that 

 one of them succeeds in reaching to the formation of the sexual 

 apparatus, as Fischer saw in Tviglochin palustre, and this, too, 

 eventually disappears. 



As regards the development of the macrospore, there is a profound 

 difference from what we have hitherto seen. The nucleus of the 

 embryo-sac divides into two ; this is twice repeated, and each group 

 of four nuclei retires to one end of the oval embryo-sac. Three at the 

 apex become surrounded with protoplasm and form naked cells — the 

 oosphere and two others termed synevgidcp. Three at the opposite end 

 also form cells which become invested with a cell-wall and are known 

 as antipodal cells; their function is unknown. The synergidae assist 

 in the process of fertilisation ; they may be regarded as " endosperm 

 cells which by an adaptation to a new function have acquired a 

 special form and place." The remaining pair of nuclei again 

 advance to the centre and join, forming a central nucleus. The 

 pollen-tube grows down the tissue of the ovary to the micropyle, 

 passes through this, and becomes attached to the embryo-sac 

 (which has occupied almost the whole of the nucellus) just above 

 the oosphere. 



Fertilisation occurs as in the Gymnosperms and the resulting 

 oospore becomes at once invested with a cell-wall. The develop- 

 ment of the embryo is on a similar plan to that in Gymnosperms, 

 and differs entirely from the mode of formation in Vascular 

 Cryptogams. 



After fertilisation, and during the early stage of embryonic 

 development, the nucleus of the embryo-sac, by repeated bipartition, 

 forms a number of free nuclei, and the sac becomes filled with endo- 

 sperm tissue. Similarly the ovule becomes the seed. 



It is evident that in Angiosperms the female sexual generation is 

 reduced as far as possible. The prothallium is represented by the 

 synergidae and, perhaps, the antipodal cells. The archegonium has 

 been reduced to the central oosphere. The endosperm formed after 

 fertilisation must probably be regarded as a new formation, and 

 not strictly comparable to the endosperm of Gymnosperms, which is 

 the true homologue of the prothallium of Vascular Cryptogams. 



