110 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



embryo sac of Gunnera were to be studied again and a sufficient 

 quantity of material examined, it would show a range of variation 

 similar to that in Peperomia. 



Penaea Type. Stephens (1909) described an interesting mode of 

 development in three genera of the Penaeaceae, viz., Penaea, 

 Br achy 'siphon, and Sarcocolla (Fig. 71). Here the 16 nuclei lie in 

 four distinct quarters which are arranged crosswise, one at each 

 end of the embryo sac and two at the sides. Now 3 nuclei of each 

 quartet become cut off as cells, while the fourth remains free and 



^i&te 



L ^rMPi 



Fig. 71. Development of embryo sac in Penaeaceae. A, Sarcocolla minor, four 

 megaspore nuclei at close of the second meiotic division. B, S. formosa, eight- 

 nucleate stage. C, S. squamosa, 16-nucleate stage, showing four groups of four 

 nuclei each. D, Penaea mucronata, mature embryo sac. {After Stephens, 1909.) 



moves to the center. There are thus four "triads" and four polar 

 nuclei. As a rule, the egg cell of the micropylar "triad" alone is 

 functional, although the others often look very similar. 



Embryo sacs of this type have since been described in several 

 members of the Malpighiaceae (see Stenar, 1937; Subba Rao, 1940, 

 1941) and Euphorbiaceae (Modilewski, 1910, 1911; Arnoldi, 1912; 

 Tateishi, 1927; and others) and in a few scattered genera belonging 

 to other families. 



Special mention may be made of the embryo sac of Acalypha 

 indica (Maheshwari and Johri, 1941), which, although similar, does 

 not entirely fit into the type described above. Up to the 16-nu- 



