56 



INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



of the two integuments. In Cytinus, a member of the Ramesia- 

 ceae, the outer integument is arrested in its development. In the 

 Salicaceae (see Schnarf, 1929) Populus tremula has a single integu- 

 ment, while P. canadensis and P. candicans also possess a weakly 

 developed inner integument which is apparently on its way to ex- 

 tinction. In the Icacinaceae (Fagerlind, 1945c) Gomphandra and 

 Gonocaryum are unitegmic but Phytocrene shows two primordia. 

 In some plants there is also a third integument or aril. 1 In Ulmus 



Fig. 39. Development of ovule of Opuntia aurantiaca. A, front view of young 

 ovule. B-D, longitudinal sections of progressively older ovules (nu = nucellus; 

 it = inner integument; oi = outer integument; / = funiculus). (After Archibald, 

 1939.) 



(Shattuck, 1905) it is said to originate by the splitting of the outer 

 integument, but in most other cases it is a new structure arising 

 from the base of the ovule. Good examples of this kind are seen in 

 Asphodelus (Fig. 40 A, B) and Trianthema (Fig. 40C). Of a differ- 

 ent origin is the "caruncle," found in several members of the Euphor- 

 biaceae, which arises by a proliferation of the integumentary cells 

 at the micropylar region (Landes, 1946). Sometimes this prolifera- 



1 In Canangium, Mezzettia, and Xylopia Corner (1949) records the presence of 

 a "middle integument" arising between the outer and inner integuments. In 

 Canangium and Xylopia, which also have an aril, the middle integument becomes 

 the fourth integument of the seed. 



