THE MEGASPORANGIUM 59 



is an excessive development of the upper portion of the integuments 

 so that the micropylar canal lies in folds over the nucellus. Rarely, 

 as in Ficus (Condit, 1932), Fouquieria (Khan, 1943), and Cyno- 

 morium (Steindl, 1945), the integumentary cells come in such in- 

 timate contact with each other that the micropylar canal is ex- 

 tremely narrow and imperceptible. 



Nucellus. 16 Depending on the extent of development of the nucel- 

 lus, ovules are called crassinucellate or tenuinucellate. 2 In the first 

 type, there is a well-developed parietal tissue and the megaspore 

 mother cell is separated from the nucellar epidermis by one or sev- 

 eral layers of cells. In the second type, parietal cells are absent 

 and the megaspore mother cell lies directly below the nucellar 

 epidermis. 3 



In the crassinucellate forms the nucellus may enlarge either by an 

 increase in the number of the parietal cells or by periclinal divisions 

 of the nucellar epidermis. In some plants like Zizyphus (Kajale, 

 1944) and Quisqualis (Fagerlind, 1941) (Fig. 41) both these processes 

 take place simultaneously. 



Several members of the Salicaceae, Nyctaginaceae, Euphorbia- 

 ceae, Polygonaceae, and Cucurbitaceae are characterized by having 

 a beak-shaped nucellus which reaches out into the micropyle. In 

 one species, Polygonum persicaria (Soueges, 1919), the beak forms 

 a very conspicuous structure protruding upward to the base of the 

 style (Fig. 42). 



The tenuinucellate forms are of two kinds: (1) those in which the 

 nucellus is short and the primordia of the integument or integu- 



16 For more detailed information on the nucellus, see Dahlgren (1927). 



2 It should be noted that the above distinction between crassinucellate and 

 tenuinucellate ovules, although convenient and useful, is not always sharp and 

 clear-cut and there are various intergradations between them. Further, both 

 types may sometimes occur in one and the same species. To mention only two 

 examples, in Butomus (Holmgren, 1913) and Ophiopogon (Maheshwari, 1934) in 

 some ovules the megaspore mother cell is situated directly below the nucellar epi- 

 dermis while in others it is separated from the latter by a wall cell. 



3 Even in those plants in which the ovules are usually tenuinucellate and devoid 

 of parietal cells, some of the cells of the nucellar epidermis may undergo one or two 

 periclinal divisions. Svensson (1925) and Dahlgren (1927) have figured this in 

 Helioptropium and Cobaea. Here the epidermal cells just above the megaspore 

 tetrad undergo a radial elongation followed by a periclinal division which may 

 give the false impression of the cutting off of parietal cells. 



