OVULES 



357 



e.s. 



Composite). This last condition is often characteristic of some 

 of the most advanced types of Flowering Plants. 



The ovules of Angiosperms (Fig. 206), which in their general 

 construction are like those of Gymnosperms, usually possess 

 two integuments (inner, Li., and outer, O.i.). A single one is 

 the rule amongst the 

 gamopetalous Dicotyle- 

 dons (Sympetalae) and a 

 few polypetalous families 

 (e.g. Umbelliferae, Salica- 

 ceae, etc.). In these latter, 

 however, the single in- 

 tegument is generally re- 

 latively robust, a fact 

 which suggests its possible 

 origin from the fusion of 

 two ; a partial union of 

 this character is, indeed, 

 seen in some members 

 of the Ranunculaceae. 1 

 The nucellus (n.), with 

 very few exceptions (e.g. 

 Bog Myrtle, Myrica), is 

 only clearly separated 

 from the integument at 

 the apex, where it forms 

 a cap of tissue extending 

 between the large mega- 

 spore (embryo sac, e.s., 

 see p. 365) and the micro- 

 pyle (m.). The part of 

 the nucellus in contact 

 with the integument can often be distinguished from the latter, 

 in sections of the ovule, by the dense contents of its cells, which 

 form a nutritive layer around the developing embryo sac. 



By far the commonest type of ovule is the anatropous one 

 (Figs. 206 and 207, C), in which the main body of the ovule is 



FIG. 206. Diagrammatic longitudinal 

 section through an anatropous ovule, 

 greatly magnified. a., antipodal 

 cells ; Ch. t chalaza ; e., egg ; e.s., 

 embryo sac; /., funicle ; I.i., inner 

 integument ; m., micropyle ; n., 

 nucellus ; O.i., outer integument ; 

 p., polar nuclei ; r., raphe ; s., syner- 

 gidse ; V.b., vascular bundle of raphe. 



1 In a few Rosaceae (e.g. Geum) there is only a single integument due 

 to failure of the inner one to develop. 



