FERTILIZATION 



149 



In 1891 Treub 12 announced the phenomenon of chalazog- 

 amy in Casuarina. He found the pollen-tube penetrating the 

 chalazal region of the ovule, instead of entering through the 

 micropyle. In this case the pollen-tube becomes associated with 

 the numerous elongated sterile megaspores, and doubtless they 

 are of service in rendering the passage easy; and later it enters 

 the antipodal region of the embryo-sac and approaches the egg- 

 apparatus from that direction (Figs. 67, 24(7). In 1893 

 Xawaschin 14 reported chalazogamy in Betula] and in 1894 

 Miss Benson 15 not only observed the phenomenon in Betula, 

 but also added Ahius, Corylus, and Car pinus to the list of 

 chalazogamic plants. In all of these cases Miss Benson ob- 

 served the tubes following a course 

 parallel with the vascular strands 

 of the raphe, thus reaching and 

 penetrating the chalaza. In Cory- 

 lus and Carpinus the tube enters 

 a more or less conspicuous caecum 

 developed in the antipodal region 

 of the sac, traverses it, and comes 

 in contact with the egg; but in 

 .1 hi us the tube traverses the nucel- 

 lus to the micropylar region above 

 the embryo-sac, and then turns 

 and enters it as though it had come 

 by way of the micropyle. In 1895 

 Xawaschin 20 added Juglans cine- 

 rea and /. regia to the list. In the 

 latter species the tube does not pass down the stylar canal or 

 traverse the cavity of the ovary, but advances through the tissue 

 of the style and of the ovary wall until opposite the insertion of 

 the single ovule that fills the ovary cavity. It then leaves the 

 ovary wall and pierces the chalaza, branching freely in the nu- 

 cellus, which is described as " veined " by tubes surrounding the 

 sac on all sides. The male nuclei discharged into the sac were 

 seen " wandering " in its cytoplasm and fusing with one of sev- 

 eral free cells that function as eggs but have not organized an egg- 

 apparatus. Recently Billings 66 has discovered chalazogamy in 

 Gary a olivaeformis, the common pecan, the details conforming 

 almost exactly to those given by Xawaschin for Juglans regia. 



FIG. 67. Casuarina suberosa. A 1 

 pollen-tube entering chalazal end 

 of embryo-sac, x 270; , stage 

 showing (TreuVs interpretation) 

 formation of endosperm before fer- 

 tilization, x 180. After TBEUB." 



