130 THE WHEAT PLANT 



synergidae degenerate, some residual debris from them remaining near 

 the ovum for a short period. 



The fusion of one of the male gametes with the ovum was observed 

 in examples collected between 30 and 40 hours after pollination : the fate 

 of the second gamete I have not been able to discover with certainty. 

 The two polar nuclei of the embryo-sac remain in contact without fusing, 

 up to near the time of fertilisation. If they fuse and unite with the 

 second male gamete they do so very rapidly, since free nuclei resulting 

 from the division of the primary endosperm nucleus are found a few 

 hours after the union of the male gamete with the ovum, and before the 

 former has lost its individuality (Fig. 104). 



Sax observed the union of the second male gamete with the fusion- 

 nucleus in " Kubanka " wheat (T. durum), but appearances in some cases 

 suggest that the double fertilisation process does not always take place, 

 at any rate in some forms of T. vulgar e. 



(i.) Endosperm. After fertilisation the ovum shows little sign of 



FIG. 103. Germination of pollen-grains, taken from the stigmas of a flower, showing 

 the transference of the gametes into the pollen-tube ( x 350). 



change for a few hours. The primary endosperm nucleus, however, 

 begins a rapid division immediately ; free nuclei containing 3-5 well- 

 defined nucleoli appear in the embryo-sac in considerable numbers, even 

 before the male gamete is lost in the ovum (Fig. 104) ; these become 

 scattered irregularly in the thin cytoplasmic lining of the embryo-sac, 

 and are more numerous in the neighbourhood of the embryo. By the 

 time the embryo is 10-15 celled, 8-10 days after pollination, a single 

 continuous layer of cells lines the embryo-sac, and endosperm cells with 

 dense non-vacuolated cytoplasm fill up the narrow pocket-like cavity 

 in which the embryo lies, walls between the cells appearing first in this 

 region. 



Six or seven days later the large vacuole of the sac is almost or entirely 

 filled with endosperm tissue which develops centripetally, the peripheral 

 cells being smaller than those nearer the centre. 



The aleuron layer is not clearly marked out from the rest of the 

 endosperm until the grain has attained about half its final size. 



(ii.) Nucellus. While the embryo is developing within the embryo- 

 sac the surrounding nucellar tissue, which at first is several cells thick, 



