262 



BOTANY OF THE LIVING PLANT 



takes place not in the preparation of the mother-cell, but during the first 

 divisions within it. The whole development is here condensed, and stages 

 seen elsewhere are entirely omitted. 



There is thus considerable latitude in the details of the nucellus, associated 

 with marked differences in its bulk. Speaking generally, the nucellus is more 

 bulky in relatively primitive types, such as the Rosaceae and Ranunculaceae, 

 and especially in the Fagales. It is less bulky in advanced types, such as the 

 Gamopetalous Dicotyledons, or the Orchids. But they often make up for 

 this by the elaborate structure of their single integument. 



In sharp contrast to this variability of the nucellus, or megasporan- 

 gium in Flowering Plants, is the dead level of uniformity shown by 

 the embryo-sac or megaspore, and its contents. So great is this that, 



FIG. 209. 



Stages in the development of the embryo-sac of Monotropa, after Strasburger. 

 See Text below, (i.-vii. x 400. viii. is less highly magnified.) 



putting aside the few exceptions that exist, one description will serve 

 for all Angiospermic Plants. It is based upon the observations of 

 Strasburger on the transparent ovules of Monotropa (Fig. 209). The 

 lowest of the three cells resulting from division of the megaspore- 

 mother-cell is the future embryo-sac (i). It enlarges at the expense 

 of the other two, which collapse, and their disorganised remains 

 appear as a cap covering the micropylar end of the sac (ii.-vi.). The 

 nucleus divides into two, which pass to opposite ends of the sac, 

 while a vacuole appears between them (ii.). They again divide (iii.), 

 with the result that two nuclei are formed at either end (iv.). These 

 again divide (v.), and the resulting nuclei arrange themselves in the 

 characteristic way common to embryo-sacs (vi.). Three are grouped 

 at either end, each surrounded by its own area of cytoplasm, limited 

 by a plasma-film. Those at the chalazal end form the antipodal 



