THE OVULE 261 



cialized parenchyma. This "central strand" has been considered possible 

 evidence of an ancestral vascular supply to the nucellus (see Anatomy 

 of the Ovule). At the base of the nucellus a few cells may become 

 somewhat thick-walled and lignified, forming a basal area below the 

 sporogenous tissue, the hypostace. It becomes more prominent as the 

 embryo sac develops. Its functional significance is unknown. The use of 

 terms applied to the various parts of the nucellus is loose and inconsist- 

 ent. "Calotte" is applied not only to the cap of tissue set apart by 

 radially arranged cells but to all tissues above the archesporial cells. 



Fig. 99. Longitudinal sections of developing crassinucellate ovules, Cercidiphyllum, 

 showing A, megaspore mother cell; B, linear tetrad of spores, the three outer 

 degenerating; C, germinated innermost spore, forming a 4-nucleate embryo sac, 

 the outer spores vestigial. {After Swamij and Bailey.) 



The evolutionary history of the nucellus is chiefly that of reduction 

 from the numerous cells of the massive type, in which the several arche- 

 sporial cells are deeply sunken, to the extreme tenuinucellate type, in 

 which a very few epidermal cells overlie one archesporial cell. Transi- 

 tional forms are frequent. 



Cells of the nucellus that lie below the epidermis and surround the 

 mother cell laterally, and later the embryo sac, are called parietal 

 cells, cover, and wall cells. Parietal cells is the best term, because they 

 are probably homologous with the parietal cells of the anther sac. 

 Unfortunately, they have also been called "tapetal" cells, but within 

 the nucellus there is probably no definitely specialized nutritive layer 

 like that of the microsporangial tissue; the endothelium is integumentary. 



Modifications of the micropylar end of the ovule are many. The 

 nucellus may project into the micropyle, as in some of the Caryo- 



