.STRUCTURE.] FORMS OF OVULES. 397 



as the sac existing within the nucleus, and as that in which 

 the embryo is developed, I know of no positive character 

 that can be assigned to it ; for I have reason to believe that 

 in Xanthium a second sac is to be found ; and to say nothing 

 of it, Osyris shows that the embryo is occasionally developed 

 outside it. Very generally it is confined to the nucleus. (See 

 the observations upon the aril at a future page) . 



The primine, secundine, and nucleus, have all an organic 

 connection at some one point of their surface. That point is, 

 in ovules whose parts do not undergo any alteration of direc- 

 tion in the course of their growth, at the base next the pla- 

 centa ; so that the nucleus is like a cone, growing from the 

 base of a cup, the base of which is connected with the hilum 

 through another cup like itself (Plate V. fig. 23.). The axis 

 of such an ovule, which Mirbel calls orthotropal, is rectilinear, 

 as in Myrica, Cistus, Urtica, &c.; and the foramen is at the 

 end of the ovule most remote from the hilum. 



But sometimes, while the base of the nucleus and that of 

 the outer sacs continue contiguous to the hilum, the axis of 

 the ovule, instead of remaining rectilinear, is curved down 

 upon itself (Plate V. fig. 26, 27.); so that the foramen, instead 

 of being at the extremity of the ovule most remote from the 

 hilum, is brought almost into contact with it. Examples of 

 this are found in Cloveworts (Caryophyllacese), Mignionette, 

 &c. Mirbel, who first distinguished these ovules, calls them 

 campylotropal. In both these modifications, the base of the 

 ovule and the base of the nucleus are the same. 



In a third class the axis of the ovule remains rectilinear ; 

 but one of the sides grows rapidly, while the opposite side 

 does not grow at all, so that the point of the ovule is gradually 

 pushed round to the base ; while the base of the nucleus is 

 removed from the hilum to the opposite extremity (Plate V. 

 fig. 16 21.): and when this process is completed the whole 

 of the inside of the ovule is reversed ; so that the apex of the 

 nucleus, and consequently the foramen, correspond with the 

 base of the ovule. Such ovules as these Mirbel terms ana- 

 tropal ; they are very common : examples may be found in 

 the Almond, the Apple, the Ranunculus, the Cucumber, &c. 

 When the base of the nucleus is thus removed from the base 



