Chapter 9 



FERTILIZATION'* 



The tip of the pollen tube, carrying the male gametes, breaks through 

 the embryo-sac wall nearly always at or beside the egg apparatus. In 

 chalazogamy, where the tube reaches the sac at or near the base of the 

 sac, it passes along the outside of the sac to the micropylar end before 

 entering (Fig. 103). In entering, it may pass through and destroy one, 

 rarely both, of the synergids; it may pass between the egg and a 

 synergid, or at one side of the group of cells. (In the Plumbago and 

 Plumbagella types of embryo sac, there are no synergids, and, in some 

 taxa, they may degenerate before fertilization. ) The male cells are 

 discharged into the sac, and the nucleus of one unites with the nucleus 

 of the egg, and the nucleus of the other unites with the two polar nuclei 

 in a triple fusion. These two nuclear fusions constitute "double fertiliza- 

 tion." Some phenomena in the formation of the zygote suggest that the 

 male cytoplasm enters into the formation of the zygote, but this is un- 

 certain. (In araucarian conifers, the male cytoplasm takes a prominent 

 part in the formation of the first cells of the embryo.) 



The time elapsing between pollination and fertilization is controlled 

 by many factors, especially structure of the st)4e, temperature, and 

 humidity. Length of the style is apparently of less importance than 

 other factors. The pollen may germinate on the stigma "immediately," or 

 only after some hours. The shortest time reported for the tube to reach 

 the embryo sac is fifteen to fort)^-five minutes after pollination. The 

 usual time is a few hours, but days, even months, may be required. In 

 the Orchidaceae, where the ovules may not be developed at time of 

 pollination, the period may be as long as six or seven months — Cym- 

 hidium. Where dormancy due to unfavorable climatic conditions inter- 

 venes, the period is several months or even a year — six months in 

 Hamamelis and thirteen in the red and black oaks. (Similar variation 

 in time from pollination to fertilization is present in the conifers — from 

 a few days to about thirteen months in Pinus and Agathis.) 



* For a detailed discussion of the cytological and genetical aspects of the fusion 

 of the male and female gametophytes and of the second male gamete with the 

 polar nuclei, the reader is referred to textbooks on cytology and genetics. 



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