THEORETICAL CONCLUSIONS 427 



remarkably regular feature, which has been commented on by vari- 

 ous authors. Long ago Thomas (quoted in Sargant, 1900) sug- 

 gested that a tissue resulting from a nuclear fusion which includes a 

 male element may perhaps be more suitable for the nourishment 

 of an embryo arising from the same mixed stock than one derived 

 from the maternal plant alone. A few years later Nemec (1910) 

 expressed a more or less similar opinion. He said that the fertiliza- 

 tion of the polar nuclei has a double function: (1) the stimulation 

 of endosperm development, and (2) the creation of a nutritive 

 tissue which is physiologically compatible with the embryo. 



According to Thomas and Nemec, therefore, the hybridity of 

 the endosperm is a method of adjusting its composition to the 

 needs of the developing plant, for otherwise the hybrid embryo 

 would be forced to depend upon the kind of food made available to 

 it by the maternal parent alone. 



Brink and Cooper (1940, 1947) have recently restated this view. 

 They point out that in the gymnosperms the female gametophyte 

 is packed with food materials which are readily available to the 

 egg both at the time of fertilization and during the maturation of 

 the embryo. In the angiosperms, on the other hand, the female 

 gametophyte is a greatly reduced structure in comparison to the 

 total mass of the ovule, and contains little reserve food at the time 

 of fertilization. Further, there is a competition for food between 

 the tissues within the embryo sac and those belonging to the nucel- 

 lus and integument. In order that the reproductive process may 

 be completed, it is necessary to have a mechanism which would 

 tip the scale in favor of the endosperm and enable it to maintain a 

 certain aggressiveness over the adjacent tissues of the ovule so that 

 it can act as an efficient intermediary for the nutrition of the em- 

 bryo. Brink and Cooper suggest that double fertilization is 

 a means of conferring upon the endosperm the physiological ad- 

 vantage of hybridity so that it has "two chances instead of one 

 (as in the gymnosperms) of receiving the genetic equipment neces- 

 sary to perform its function." 



This is an interesting hypothesis. Some previous writers have 

 also expressed the view that the vigor of the endosperm and its 

 parasitic relation to the nucellus might be attributed to its triploid 

 chromosomal constitution. However, the question arises as to why 

 the endosperm with its higher chromosome numbers should then 



