FERTILIZATION 



209 



Johansen (1931a, b) has reported similar occurrences in Taraxia 

 ovata and Zauschneria latifolia. Sooner or later, however, such 

 embryos are likely to stop growth so that no viable seeds are pro- 

 duced. 



The second of the two alternatives — i.e., the 

 occurrence of triple fusion without an accom- 

 paniment of syngamy— has been reported in 

 several plants, but the ovules soon begin to de- 

 generate. If seeds are formed, they are with- 

 out embryos and therefore nonviable. Rarely, 

 however, the unfertilized egg may develop into 

 a haploid embryo. Such cases will be discussed 

 in connection with apomixis (see Chap. 9). 



Persistence and Possible Haustorial Func- 

 tion of Pollen Tube. Usually the pollen tube 

 collapses soon after fertilization, and there is 

 little evidence of it after the embryo has com- 

 menced its development. There are a few cases 

 on record, however, in which it has been known 

 to persist for longer periods. In Galinsoga 

 ciliata (Popham, 1938) it is recognizable up to 

 the seven-celled stage of the embryo, and in 

 Ulmus americana (Shattuck, 1905) up to the 

 20-celled satge. In Hicoria pecan, according to 

 Woodroof (1928), it persists for two to three 

 weeks, and in one ovule he saw the dead end of 

 the tube beside the fertilized egg even seven 

 weeks after pollination. 



Cook (1909) noted a very peculiar behavior 

 of the pollen tube in Passi flora adenophylla. 

 Although fertilization stages were frequent, in 

 the majority of ovules the pollen tube did not 

 discharge its contents but continued its growth 

 within the embryo sac, becoming greatly twisted 

 and tangled in the process. Its growth was 

 sometimes so vigorous that all the contents of 

 the sac, including the egg apparatus, were com- 

 pletely absorbed. A few years later, Cook 



Fig. 118. L.s. embryo 

 sac of Zostera marina, 

 showing young em- 

 bryo and undivided 

 secondary nucleus. 



(1924) noted a similar phenomenon in an ovule (After Dahlgren,1989.) 



