THE OVULE 269 



Nymphaeaceae. The orthotropous ovule is found chiefly in the more 

 highly specialized families where placentation is basal and the ovule 

 solitary— Juglandaceae, Polygonaceae, Piperaceae, Restionaceae, Naja- 

 daceae. The campylotropous ovule characterizes some prominent fam- 

 ilies — Capparidaceae, Caryophyllaceae, Geraniaceae, Apocynaceae, 

 Verbenaceae. Families may be uniform in ovule type or may have 

 more than one type; the Palmae have both anatropous and ortho- 

 tropous ovules. The direction of curvature of individual anatropous 

 ovules may vary, depending upon space available. Occasionally, more 

 than one type of ovule may occur even in the same carpel. Changes may 

 occur in ovule type in later stages; an ovule anatropous at pollination 

 may become campylotropous after fertilization, as it becomes adnate 

 to the funicle. 



Variety of ovule form doubtless represents adaptation of an ancestral 

 type to conditions within the ovary — number of ovules, available space, 

 placental type, location of pollen-tube transmitting tissue. All these 

 conditions have doubtless been important in the modification of an 

 ancestral form. The important problem is the determination of this 

 primitive form. Modifications in form in minor ways are sometimes re- 

 lated to lack of space for normal development, but lack of space plays 

 no important part in the determination of shape, because all types 

 occur where the ovarian chamber is large, and beginnings of inversion 

 occur in early stages. Relation to course of the pollen tube may in- 

 fluence ovule orientation in highly specialized ovaries where the ovule 

 is solitary. Ancestral form probably plays the important part in ovule 

 type. Both the orthotropous and the anatropous types have been con- 

 sidered primitive. The orthotropous ovule was perhaps selected as 

 primitive because of its simplicity of form, but its correlation with 

 high specialization in the carpel, gynoecium, and flower seems to show 

 that its simplicity is derived. The anatropous type characterizes the most 

 primitive families of both dicotyledons and monocotyledons. It is typi- 

 cally crassinucellate, and its archesporial tissue is frequently multicel- 

 lular. The inversion of the anatropous ovule has been interpreted as 

 evidence of specialization, as an adaptation to the course of the pollen 

 tube, a placing of the micropyle closer to the carpellary surface along 

 which the pollen tube grows. On the other hand, resemblance of this 

 ovule type has been seen to fertile psilophytalian telomes. (A simi- 

 lar inversion of ovules is present in Paleozoic conifers.) But the ovule 

 is a complex structure, and it seems most unlikely that the simple 

 telomes of the early land plants should persist in the angiosperms. The 

 anatropous ovule seems to be the basic type, one inherited from 

 ancestral stock. The dominance of this type and its presence in the most 

 primitive orders of both dicotyledons and monocotyledons is strong 



