ARCHESPORIUM 



291 



those with highly speciaHzed ovules or gynoecia, only one is usually 

 formed. Where there are several of these cells, they may form a plate, 

 a cluster, or a longitudinal row. Larger numbers of sporogenous cells 

 — some of which may not mature as spore mother cells — and of parietal 

 cells doubtless represent primitive ovule structure; the solitary sporog- 

 enous cells and one or no parietal cells, the advanced structure. Within 



Fig. 111. Longitudinal sections of young crassinucellate ovule of Casuarina montana 

 showing ontogeny of multicellular archesporium, parietal cells, and free integuments. 

 A, early stage, sporogenous cells with large nuclei; lower cells still maturing; 

 parietal cells formed by tangential divisions. B, later stage, megaspores mature, some 

 germinating; procambial strand at base of ovule. {After Swamy.) 



families, there are occasional marked differences in amount of parietal 

 tissue; in the Ranunculaceae, parietal tissue may be abundant or scanty; 

 in the Rosaceae, genera with achenes — Fragaria — may have massive 

 nucelli. Few or no parietal cells characterize most of the gamopetalous 

 taxa; massive nucelli are largely restricted to the lower polypetalous 

 taxa. 



