HISTORICAL SKETCH 



17 



tive or tube nucleus remains undivided. Thus, the pollen tube 

 eventually shows three nuclei, one vegetative and two generative 3 . 



On the basis of his studies on the embryo sac of Monotropa and 

 some other plants, Strasburger further showed that the pollen tube 

 discharges its nuclei into the sac (previous to this it was believed 

 that fertilization occurred merely by the diffusion of the cell sap 

 from the tube) and that one of the two male nuclei fuses with the 

 nucleus of the egg, thus provid- 

 ing actual proof of the nature "™ 

 of fertilization and its import- 

 ance in the life cycle of a plant 

 (Fig. 14). 



In the concluding part of his 

 memoir, Strasburger made the 

 following generalizations, which 

 are now almost axiomatic with 

 us: (1) the process of fertilization 

 comprises the union of the nu- 

 cleus of the male gamete with 

 that of the egg; (2) the cyto- 

 plasm of the gametes is not con- 

 cerned in the process ; and (3) the 

 sperm nucleus and the egg nu- 

 cleus are true nuclei. 



Chalazogamy. Strasburger's 

 work opened the way to a more 

 detailed study of the process 

 of fertilization in angiosperms. 

 Prior to 1891, it was believed that 



the pollen tube always enters the ovule through the micropyle. 

 Treub, in that year, reported that in Casuarina it enters through the 

 chalaza (Fig. 16). This was thought to be so strange that he proposed 

 a new classification of the angiosperms into two classes : the chala- 

 zogams and theporogams, with Casuarina as the only representative 

 of the former. Later investigations showed, however, that there is no 

 uniformity in the mode of entry of the pollen tube into the embryo 

 sac, and in Ulmus (Nawaschin, 1898a) its behavior was found to be 

 particularly varied and irregular. The phenomenon of chalazogamy, 



'These two are now called the male gametes. 



Fig. 15. Melchior Treub. (Photo- 

 graph obtained through the courtesy of 

 Dr. F. Verdoorn) 



