266 MORPHOLOGY OF THE ANGIOSPERMS 



ports other evidence that this family is more closely related to poly- 

 petalous than to other gamopetalous families. This view of the Cucur- 

 bitaceae is now frequently expressed. 



Nucellar type is fairly constant in families — probably more frequently 

 so than ovule type — but cannot be used generally as a family character; 

 some families have more than one type of nucellus — Araliaceae, Corna- 

 ceae, Saxifragaceae, Ranunculaceae, Dilleniaceae, Caryophyllaceae, 

 Potamogetonaceae, Araceae, Liliaceae, Amaryllidaceae, Iridaceae, Gra- 

 mineae. 



As the embryo sac and the embryo develop, the nucellus is pro- 

 gressively destroyed. The disappearance, in some taxa, of the small, 

 few-celled nucellus during early stages of sporogenesis has caused mis- 

 interpretation of tissues in the micropylar region, especially in fruits. In 

 massive nucelli in which a "calotte" (see Ontogeny of the Ovule) is 

 present, this cap may persist through early and even late stages of 

 embryo development, as in Elaeagmis and Vitis. Though the nucellus 

 is usually wholly destroyed during embryo development, it may persist 

 in the seed as perisperm. 



A suitable definition of the angiosperm ovule is difficult to make, and 

 definitions vary greatly. "Ovule," like "seed," is necessarily a loosely 

 used term. Is the presence of sporogenous cells, or of an embryo sac, the 

 essential character? Does the ovule become a seed after fertilization? 

 Structurally and ontogenetically, the ovule is an emergence of the 

 carpellary lamina, within which megasporogenous tissue and the embryo 

 sac, a female gametophyte, are borne. The morphological nature of this 

 emergence is still uncertain — the nucellus is probably not the mega- 

 sporangium, as commonly interpreted. Some definitions require the 

 presence not only of an integument but of a gametophyte. But, as a 

 recognizable organ, an ovule primordium, the ovule exists before integu- 

 ments develop and before a spore is present. 



Form and Orientation of the Ovule 



Variations in general form and in position of micropyle distinguish 

 several types, fundamentally much alike and representative of evolu- 

 tionary modifications of a basic type. Transitional forms are frequent. 

 The ovule is orthotropous where it is straight and upright on the 

 placental surface, the micropyle distal, and the funicle short or absent; 

 anatropous where it is reflexed at the chalazal region and appressed to 

 or adnate to the funicle and the micropyle usually faces the placenta; 

 campylotropous where the ovule is more or less bean-shaped and the 

 micropyle faces the placenta and is attached near the middle of the 

 ovule; amphitropotis where the funicle is adnate to the ovule proper 

 for about half its length and the micropyle faces laterally. The term 



