96 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



times the last division fails to occur at this end of the embryo sac 

 (Fig. 6 ID), so that only 6 nuclei are formed (Fig. 61 E). 



Oenothera Type. About a hundred years ago, Hofmeister (1847, 

 1849) published some remarkably accurate figures of the embryo 

 sac of a few members of the Onagraceae, but because of the crude 

 technique of those days he was unable to give a full account of the 

 development. Geerts, in 1908, found that in Oenothera lamarckiana 

 the embryo sac is usually formed by the micropylar megaspore of 

 the tetrad, which undergoes only two nuclear divisions instead of 

 the usual three occurring in the Polygonum type of embryo sac. 

 In this way, 4 nuclei are produced which organize into the two 

 synergids, the egg, and a single polar nucleus. Since the third 

 division is omitted and all the nuclei are situated in the micropylar 

 part of the developing embryo sac, there is neither a lower polar 

 nucleus nor any antipodal cells. Modilewski (1909) independently 

 studied species of Oenothera, Epilobium, and Circaea and confirmed 

 the observations of Geerts in all essential respects. These two in- 

 vestigations were soon followed by several others and this mode of 

 development, known as the Oenothera type, has been found to be a 

 characteristic and constant feature of the entire family Onagraceae, 

 having been demonstrated in more than 16 genera. The only ex- 

 ception is Trapa, which has an 8-nucleate embryo sac of the 

 Polygonum type, but this genus, as most systematists now agree, 

 is best assigned to a separate family, the Hydrocaryaceae or 

 Trapaceae. 



A noteworthy feature in the development of the Oenothera type 

 of embryo sac is the concurrent growth of more than one cell of the 

 tetrad. Eventually it is the micropylar megaspore which func- 

 tions, but sometimes it may be the chalazal and occasionally both 

 grow simultaneously forming "twin" embryo sacs (Fig. 62). 



Rarely, more than 4 nuclei may be seen in an embryo sac. Usu- 

 ally this condition results from the incorporation of an adjacent 

 megaspore and its contents, but it appears that sometimes there 

 may be further division or divisions of the nuclei of the embryo 

 sac. In Anogra pallida 7 Johansen (1931a) reported repeated ami- 



7 This plant is a native of the arid regions of southern Arizona and California. 

 It shows little or no seed production, and propagation occurs by means of offshoots 

 at the ends of subterranean stolons. 



