194 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



illegitimate pollinations more than 72 hours are required and fre- 

 quently fertilization fails altogether. 2 



A point of considerable interest is that while compatible pollen 

 tubes grow faster at higher temperatures, the incompatible tubes 

 show a further decline in their growth rate. This seems to indicate 

 that incompatibility is probably due to a chemical reaction similar 

 to an antigen-antibody reaction in animals, and that its speed, like 

 that of all chemical reactions, is increased by a rise in temperature 

 (Lewis, 1948). 



Gametic Fusion. After the pollen tube has discharged its con- 

 tents into the embryo sac, one male gamete fuses with the egg 

 (syngamy) and the other with the two polar nuclei (triple fusion). 

 Because of the technical difficulties encountered in studying the 

 process, very few detailed accounts of it have appeared up to this 

 time. The time between the beginning and the end of the gametic 

 fusions is so short that one rarely succeeds in "catching" the material 

 at the right stage. There is also an element of chance in obtaining 

 proper median sections, for the embryo sac is usually large enough 

 at this stage to run into several sections, and thick sections do not 

 stain satisfactorily. Besides, even if the material has been properly 

 selected and the desired stages are actually at hand, detailed ob- 

 servations may still prove difficult for the following reasons: (1) 

 the pollen tube discharges a deeply staining material into the embryo 

 sac which surrounds the egg and decreases visibility; (2) one or 

 both of the synergids disorganize at this time and their contents 

 become converted into a tenacious mucus-like material which stains 

 densely; and (3) the vegetative nucleus (or its fragments) and the 

 nuclei of the synergids "wander" into the upper part of the embryo 

 sac and are liable to be confused with the male gametes. 



In view of these difficulties it is not surprising that our knowledge 

 of the events concerned with fertilization has not advanced very 

 far beyond where it stood during the early part of this century. 

 Several workers have confessed with a feeling of disappointment 

 that, in spite of repeated efforts and the study of hundreds of ovules, 

 they failed to find many of the critical stages in the process. 



The present status of the subject may be dealt with in two parts : 

 (1) form and structure of the male gametes, and (2) details of the 



2 For further information on this topic, see Ernst, 1936. 



