SPERMATOPHYTES 



261 



the ovules may be foliar or cauline. In the different groups of angio- 

 sperms, however, the ovules are borne in very definite ways. 



General structure. — In the development of the ovule, the nucellus 

 first appears as a protrusion from the surface which bears it; later a ring 

 arises around its base, which develops into an integument; and still 



Fig. 581. — Development of an anatropous ovule, the series beginning at the left; the 

 two integuments appear successively and gradually overtop the nucellus as the ovule 

 becomes curved; last figure a section showing relation of the two integuments and the 

 nucellus at maturity of ovule. — After Gray. 



later a second ring may arise outside of the first, which develops into a 

 second integument (fig. 581). Soon the integument (or integuments) 

 overtops the nucellus, and where it closes in over the nucellus there is 

 left a narrow, more or less elongated passageway, the micropyle (fig. 

 582). Among the Archichlamydeae and monocotyledons there are 



m 

 em-, 



Figs. 582-584. — Directions of ovules; 582, orthotropous; 583, campylotropous; 

 584, anatropous; also showing outer {p%) and inner {ii) integuments, micropyle (m), 

 nucellus («), and embryo sac {em). — After Coulter. 



usually two integuments; while among the Sympetalae there is almost 

 invariably a single massive integument. 



Direction. — Important differences are shown in the directions as- 

 sumed by mature ovules. Some grow straight outward from their place 

 of origin, the axis being straight and the micropyle directed away from 

 the point of origin; such ovules are called orthotropous (fig. 582), and this 

 condition is regarded as the most primitive. In other ovules the axis 



