EMBRYOLOGY IN RELATION TO TAXONOMY 365 



6. The mature embryo sac is eight-nucleate and arises from the 

 chalazal cell of a linear tetrad of megaspores. 



7. The endosperm becomes cellular at a very early stage. 



8. The fertilized egg forms a long, tubular, somewhat vermiform 

 structure designed to penetrate rather deeply into the endosperm. 



From a consideration of the sum total of these characters, Hallock 

 concludes that the Garryaceae are not primitive but must be con- 

 sidered as the highest of the Umbelliflorae immediately preceding 

 the Sympetalae. She further suggests that the dioeciousness of 

 Garry a may also be a derived rather than a primitive feature. 



One point which has not been satisfactorily settled by Hallock 

 but which is nevertheless of considerable importance, is the nature 

 of the rudimentary haustorial structures observed by her at both 

 ends of the endosperm. She considers them to be derived from the 

 synergids and the antipodal cells, but her illustrations do not seem 

 to prove this interpretation. It is more likely that they really 

 originate from the micropylar and chalazal cells of the endosperm. 

 If this interpretation turns out to be correct, it would form one of 

 the strongest arguments in favor of the advanced position of the 

 Garryaceae. 



Onagraceae. The family Onagraceae affords one of the best 

 examples of the utility of embryological characters in taxonomic 

 considerations. An unfailing characteristic of this family, ocurring 

 in every genus and species so far investigaged, is the peculiar raono- 

 sporic four-nucleate embryo sac, consisting of an egg apparatus and 

 single polar nucleus. The only exception is the genus Trapa. This 

 has been placed variously by different systematists : (1) under the 

 Onagraceae; (2) as an appendix to the Onagraceae; (3) as an iso- 

 lated member of the Halorrhagidaceae ; (4) as the only genus of a 

 separate family, Hydrocaryaceae or Trapaceae. 



Embryological evidence strongly favors the last view. In addi- 

 tion to its eight-nucleate embryo sac, Trapa has a well-developed 

 suspensor haustorium (Fig. 200), both these features being unknown 

 in any member of the Onagraceae. Further, in the Onagraceae the 

 ovary is inferior and tetralocular, with axile placentae bearing nu- 

 merous ovules, and the fruit is generally a loculicidal capsule. In 

 Trapa, on the other hand, the ovary is semi-inferior and bilocular, 

 with only one ovule in each chamber, and the fruit is a large one- 

 seeded 5 drupe whose fleshy layer soon disappears, leaving only the 



6 Of the two ovules in the ovary, one aborts at an early stage in its development. 



