222 



INTRODUCTION TO EMBRYOLOGY OF AN GIO SPERMS 



Nuclear and the Cellular types. Here the first division is followed 

 by a transverse wall resulting in a micropylar and a chalazal cham- 

 ber. Subsequent divisions are usually free 

 nuclear and may take place in both chambers, 

 but as a rule the main body of the endo- 

 sperm is formed by the micropylar chamber 

 only (Fig. 139). 



Nuclear Endosperm. Usually at least the 

 first few divisions are synchronous, but in 

 later stages some of the nuclei may be seen 

 in the prophase stage, others in metaphase, 

 and still others in anaphase or telophase. 3 

 Thus the number of endosperm nuclei may 

 not always be a multiple of two. As divi- 

 sions progress, the nuclei become pushed more 

 and more towards the periphery, so that the 

 center is occupied by a large vacuole. Often 

 the nuclei are especially aggregated at the 

 micropylar and chalazal ends of the sac and 

 form only a thin layer at the sides. An in- 

 teresting condition has been reported in Musa 

 errans (Juliano and Alcala, 1933), where some 

 of the endosperm nuclei divide more actively 

 than others, forming isolated groups or ''nod- 

 ules." They become invested with a distinct 

 cytoplasmic wall and extend into the center of 

 the embryo sac, developing as separate endo- 

 sperm masses (Fig. 122). Similar nodules and 

 vesicles have been seen in a few other plants, 

 but their further fate and function have not 

 been clarified. In Isomeris arborea (Billings, 

 1937) they are said to give rise to embryos, 

 but this deserves confirmation. 



Frequently the endosperm nuclei in the 

 chalazal part of the embryo sac have been 



3 Nuclei in different stages of division are not indis- 

 criminately scattered, however. They often show a 

 remarkable gradation, presumably due to the influence 

 of some kind of slowly diffusing hormonal substance 

 (see Dixon, 1946). 



Fig. 119. Embryo sac 

 of Crepis capil'aris, show- 

 ing diploid chromosome 

 complement (2n = 6) in 

 cells of embryo and trip- 

 loid chromosome comple- 

 ment (3n = 9) in cells of 

 endosperm. (After (Jeras- 

 simova, 1933.) 



