THE ANGIOSPERMAE 141 1 



It is an open question, though one of some importance, whether the 

 nuclei of the embryo sac are quahtatively different, each having a predeter- 

 mined part to play, or whether their fate is determined by the position they 

 occupy in the sac. The details of embryo sac development have been fol- 

 lowed in a very large number of plants, but they do not provide a sufficient 

 answer to what is really a question of genetic constitution. Support for the 

 first view may be found in the observation that the synergidae appear to be 

 always sister cells and that the oosphere and the upper polar nucleus are 

 also sisters. Similarly among the antipodals, though the question of 

 differentiation does not usually arise, the two lower nuclei are sisters and the 

 upper antipodal is the sister of the lower polar nucleus. Differences of size 

 between the nuclei are not reliable as an index of differentiation, since this 

 varies considerably during the maturation of the sac. 



Once the eight nuclei have taken up their characteristic positions, cell 

 formation follows, generally at both ends simultaneously, but w^here there 

 is any difference it has always been observed that the antipodal cells are 

 formed before the micropylar cells. On the mode of wall formation there 

 are relatively few observations. In Lilitini martagon and one or two other 

 plants, spindle fibrils in the cytoplasm have been observed between the four 

 micropylar nuclei, on which cell plates form between all four nuclei simul- 

 taneously, so that three complete cells are enclosed, while the polar nucleus 

 is only partly enclosed, on the side towards the oosphere. The same process 

 occurs in the antipodal group. This recalls the simultaneous wall formation 

 between the nuclei of a nuclear endosperm (see p. 1453). 



Cell wall formation is suppressed in some wild species of Tiilipa. The 

 eight nuclei remain free in the cytoplasm of the sac. The synergid and the 

 oosphere nuclei have a distinct appearance but the others are indeterminate. 

 This curious anomaly is probably derived from the Fritillaria type of 

 embryo sac. 



The Synergidae. The normal pair are usually apparently similar and 

 equivalent. The name given them by Strasburger is generally justified in 

 that they assist in fertilization, after which their usefulness seems to be over 

 and they disappear. They may, indeed, disappear before fertilization, but 

 this is exceptional. Their pointed upper ends are attached to the apex of 

 the embryo sac and their rounded lower ends protrude into the space of the 

 sac, reaching from one-fourth to one-third of its length (Fig. 1302). A few 

 cases are known {e.g.. Luff a) where they are so much enlarged that they reach 

 nearly to the base of the sac. 



The walls of the synergidae contain cellulose, that is they are true cell 

 walls. There is frequently a definite constriction of the wall, separating an 

 upper and a lower part of each cell,* and above this the cells expand laterally, 



* The terms " upper " and " lower " have been used here throughout in accordance with 

 the older usage, which regards the micropylar end of the ovule as its apex and therefore the 

 point of reference in orientation. Moreover all illustrations of embryo sacs are drawn in this 

 sense. The practice of some recent writers of using the terms in reverse, taking the orientation 

 of the embryo for their guide, is to be deplored, as introducing confusion and as illogical when 

 applied to structures before the embryo exists. It is no improvenient; on the contrary. 



