THE OVULE 



263 



Apocijmtm, one of the highest genera, the nucellus persists only as a 

 few covering cells; the sheathing layer is no longer distinguishable (Fig. 

 lOlC, D). This t}^pe, described as "vestigial," characterizes tlie Asclepi- 

 adaceae (Fig. lOlE) and many of the Sympetalae. The Commelinaceae 

 have been reported to have a "filamentous and constricted" nucellus, 

 consisting of a small uniseriate row or cluster of cells surrounded by an 

 epidermis. In the orchids, the nucellus (Fig. 102) has been called "in- 

 significant," and interpretation of its structure is difficult. The presence 

 of hypodermal cells here is in doubt, as is the reported epidermal origin 



Fig. 101. Sketches of reduced nucelli in the Apocynaceae. A, Cerbcra lactaria, 

 nucellus, a uniseriate sheath; B, Vinca rosea, lateral cells of nucellus crushed by 

 integument; C, V. minor; D, Ajwci/num cannabinum; E, Asclcpias curassavica. 

 C, D, E, lateral cells of nucellus absent, distal cells continuous with epidermal 

 cells of integument. (After Guignard.) 



of the spore mother cells. In taxa where no ovule is formed, as in the 

 Balanophoraceae and Loranthaceae, cells of the placenta or even of the 

 ovary wall function as a nucellus. In Arceuthobiiim piisillum, where 

 there is no ovule, tliere is only a hump of undifferentiated placental 

 tissue, in which two embryo sacs develop (Fig. 108E, F). Even in well- 

 developed ovules, a nucellus may be absent at flowering time, as in 

 Paeonia and Nanditm, where the nucellus is whollv absorbed before 

 pollination and an integumentary nutritive layer becomes palisadelike 

 (a "mantle tissue"). 



In the massive nucellus of some amentiferous genera, a central core of 

 tissue with a stalklike base is set apart histologically. Within this core, 

 the sporogenous tissue develops. This core has been considered possible 



