174 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



generative cells have been differentiated, further development is 

 quite normal and no pollen-embryo sacs are formed. Further, the 

 pollen-embryo sacs were always accompanied by a large number of 

 dead pollen grains, leading Stow to suggest that the latter secrete a 

 "necrohormone" which causes an abnormal growth of the surviving 

 pollen grains. 



Stow also observed that when the pollen-embryo sacs were placed 

 on an agar medium, together with some normal pollen grains of 

 another variety, the pollen tubes formed from the latter coiled 



B 



Fig. 106. Fertilization of pollen-embryo sacs in Hyacinthus orientalis. A, normal 

 pollen grain, showing vegetative and generative cells. B, pollen-embryo sac. 



C, pollen-embryo sac affected by pollen tube from pollen grain of another variety. 



D, fertilized pollen-embryo sac; the smaller nuclei are presumed to be products of 

 division of triple fusion nucleus. (After Stow, 198/,.) 



around the former (Fig. 106). Once a sperm nucleus was observed 

 to be in process of entering the pollen-embryo sac; and in another 

 case the pollen-embryo sac showed 16 nuclei, believed to have been 

 derived from the divisions of a triple fusion nucleus. 



In conclusion Stow says that all pollen grains are potentially 

 capable of assuming either the male or the female form. Under 

 normal conditions the "male potency" is dominant over the "female 

 potency" leading to the formation of the generative cell and the 

 male gametes; but under abnormal conditions, when there is a re- 

 lease of necrohormones, the female potency gets the upper hand 

 resulting in the formation of embryo-sac-like structures. 



