302 MORPHOLOGY OF THE ANGIOSPERMS 



The most strongly supported theory is that the sac consists of two 

 archegonia (axial cell rows only), without prothallial cells. The micro- 

 pylar cluster ( the egg, the ventral canal cell or micropylar polar nucleus, 

 and the two synergids) and the chalazal cluster, a similar group, but 

 with a nonfunctioning egg, are considered to represent two four-celled 

 archegonia, located at the two ends of the sac. In support of this theory 

 is the definite arrangement of the nuclei in polar clusters of four nuclei 

 or cells — even in most of the specialized types — and the occasional "in- 

 verted polarity," where the functions of the clusters are reversed and 

 the chalazal cluster contains the egg. It has been pointed out, in refuting 

 this interpretation, that reduction of the archegonium in other taxa is 

 by progressive shortening of the "axial row," — the neck cells lost first, 

 then the ventral canal cell, while the jacket and prothallial cells 

 still persist. Under both these theories, it is apparent that the female 

 gametophyte in the angiosperms is reduced to an egg and several 

 associated cells, whatever their morphological nature may be. 



A modification of the two-archegonia theory interprets the micropylar 

 cluster of four cells as remnants of two archegonia and the chalazal 

 cluster of four cells as wholly prothallial. The egg and one synergid is 

 said to represent one archegonium; the polar nucleus and the other 

 synergid, a second archegonium. In support of this theory is the inter- 

 pretation of the fusion of the micropylar polar nucleus with a male 

 nucleus as a true fertilization, which gives rise to endosperm. In refuta- 

 tion of this two-archegonia-at-one-end theory is the assumption ( 1 ) that 

 the egg and the one synergid are sister cells and (2) that the formation 

 of the primary endosperm nucleus is an act of fertilization, because a 

 male nucleus enters into the triple fusion. The egg and polar nucleus 

 have been shown to be one pair of sister nuclei and the synergids an- 

 other pair. The triple fusion does not produce an embryo. 



Another interpretation of the embryo sac is based on comparison of 

 the sac with that of Gnetum. In Gnetum, the sac contains many free 

 nuclei, all of which appear to be potentially eggs. The "gnetalean 

 theory" assumes that the angiosperm sac is derived from a sac of this 

 kind by reduction in number of nuclei. But the Gnetum sac also appears 

 to be a highly modified gametophyte, specialized by the multiplication, 

 rather than the reduction, of potential eggs. In angiosperms, the syner- 

 gids have been called "new features, not present in the gymnosperms," 

 just as double fertilization has been called a new feature. If the embryo 

 sac of Gnetum provides a stage intermediate between that of most 

 gymnosperms and that of angiosperms, it should show some evidence in 

 transitional form, rather than an entirely different type of gametophyte. 

 The angiosperm gametophyte is so greatly reduced that, apparently, no 

 interpretation of its nature can be well supported, even if the sac is 



