262 



MORPHOLOGY OF THE ANGIOSPERMS 



phyllaceae and species of Polygonum, or even extend beyond it, as in 

 Rhodotijpus, Malva, Hibiscus, some species of Euphorbia. These pro- 

 liferations of the nucelhis may fuse with the integuments or with the 

 obturator, as in Manihot. The distal epidermal cells of the nucellus may 

 become greatly elongated and prominent, or develop thickened outer 

 walls. Though the nucellus is always terminal, it may not appear so in 



Fig. 100. Longitudinal sections of tenuinucellate ovules showing megasporogenesis, 

 Lobelia cardinalis. A, ovule primordium showing hypodermal archesporial cell; B, 

 archesporial cell enlarged, integument developing; C, first meiotic division in 

 megaspore mother cell; D, daughter cells of megaspore mother cell; E, linear tetrad 

 of megaspores, the proxmial enlarging; F, the proximal spore mature, the others 

 aborted. {After Cooper.) 



tenuinucellate forms, where it consists of very few cells and is over- 

 topped by a massive integument. 



An important step in the reduction of the nucellus is seen in the 

 Apocynaceae. In the more primitive taxa, the nucellus consists of a 

 uniseriate sheath about the spore mother cell and the spore tetrad 

 (Fig. lOlA); in some genera, the lateral, sheathing (parietal) cells 

 collapse and disappear, so that the nucellus is reduced to a single layer 

 of a few cells on the top, directly below the micropyle (Fig. lOlB). In 



