112 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



number. These counts could not be regarded as certain, however, 

 since such embryo sacs ran into three or four sections and their 

 exact reconstruction was a matter of doubt. 



Abnormalities of a somewhat similar nature have also been re- 

 corded in Combretum (Mauritzon, 1939), but it has to be seen how 

 far these are related to differences of environment. 



Drusa Type. A 16-nucleate embryo sac of a different nature was 

 recorded by Hakansson (1923) in Drusa oppositifolia, a member of 

 the family Umbelliferae (Fig. 73 A). After the meiotic divisions 

 are over, three of the megaspore nuclei pass down to the basal end 

 of the embryo sac, and only one remains at the micropylar end. 

 This 1+3 arrangement is followed by a 2+6 and then a 4 + 12 

 stage. The four micropylar nuclei give rise to the egg apparatus 

 and upper polar nucleus, and the 12 chalazal nuclei to a lower polar 

 nucleus and 11 antipodal cells. 



During recent years this type of development has been recorded 

 in Mallotus japonicus (Ventura, 1934), Maianthemum bifolium and 

 M. canadense (Stenar, 1934; Swamy, 19496 9a ), Crucianella latifolia, 

 Rubia olivieri (Fagerlind, 1937), Tanacetum vulgare, Chrysanthemum 

 parthenium (Fagerlind, 1941), Ulmus (Ekdahl, 1941; Walker, 1950), 

 and a few other plants. A few of these deserve special mention 

 and are briefly discussed below. 



Shattuck (1905) reported an 8-nucleate embryo sac of the Adoxa 

 type in Ulmus americana, but he observed that frequently there 

 seemed to be a further nuclear division. Several embryo sacs were 

 found to contain as many as 12 or more nuclei, rather evenly dis- 



9a In M. canadense, according to Swamy (19496), in about 13 per cent of the 

 ovules the chalazal spindles of the last division fuse in pairs so that the mature 

 embryo sac comes to possess 4 haploid nuclei at the micropylar end and 6 diploid 

 nuclei at the chalazal end. 



Pig. 73. Development of embryo sac in Drusa oppositifolia (A), Chrysanthemum 

 parthenium (B-II), and Crucianella latifolia (I-M). A, Drusa, 16-nucleate embryo 

 sac, showing four nuclei at micropylar end and twelve at chalazal. (After Hakans- 

 son, 1928.) B,C, Chrysanthemum, young embryo sacs showing varying arrange- 

 ments of the four megaspore nuclei. D-F, eight-nucleate stage; note degeneration 

 of basal nucleus in E. G, last division in embryo sac, basal nucleus degenerating. 

 //, mature embryo sac, showing 12 nuclei. (After Fagerlind, 1941.) I, Crucianella, 

 megaspore nuclei. J, same, in division. K, fourth division in embryo sac; some 

 of nuclei at chalazal end have failed to divide. L, embryo sac, showing 15 nuclei. 

 M, mature embryo sac. (After Fagerlind, 1937.) 



