OVULE. 



[ 480 ] 



OVULE. 



ment. Some term them the integumentum 

 internum and externum. The inner is the 

 tegmen, the outer the testa of R. Brown. 

 The passage at the apex, leading to the 

 nucleus, is called the micropyle ; sometimes 

 the orifice in the outer coat is distinguished 

 from that in the inner coat, and they are 

 termed respectively exostome and endostome 

 (fig. 552). While the nucleus and coats are 

 becoming perfected, one of the cells situated 

 near the apex of the nucleus takes on a pecu- 

 liar character, becoming more developed than 

 the rest, and often causing the absorption of 

 part (or sometimes the whole) of the tissue 

 of the nucleus ; it appears at length as a 

 large sac occupying the centre of the ovule ; 

 this is the embryo-sac (fig. 549). The base 



Fig. 549. 



Sections of atropous ovule of Polygonum. 



P, primine ; S, secundine ; N, nucleus ; SE, em- 

 bryo-sac ; V. e, PI, nascent embryo. 



Magnified 20 diameters. 



of the ovule is pushed up from the surface of 

 the placenta during its development so as to 

 appear at length supported on a stalk of vari- 

 able length, this is termed ihefuniculus (figs. 

 548 F, 552/) ; the point of attachment of 

 this stalk to the body of the ovule (marked by 

 a scar when the ripe seed separates) is called 

 the hilum. That region of the interior where 

 the lower parts of the coats are confluent 



551. 



Section of campylitropous ovule of the wallflower. 

 C, chalaza ; N, nucleus ; S, inner coat ; P, outer coat. 



Magnified 20 diameters. 



with the base of the nucleus, is called the 

 chalaza (fig. 551 C). 



The form of ovules is much affected by 

 excessive development of its constituent parts 

 in special directions before the fertilization. 

 If all parts grow equally, the complete ovule 

 is erect on the placenta, with its hilum and 

 also the chalaza turned towards the latter, 

 and its micropyle at the opposite free end ; 

 such an ovule is technically termed atropous 

 or orthotropous (figs. 546-550). Very fre- 



Fig. 553. 



Fig. 554. 



Magnified 40 diameters. 



Fig. 555. Fig. 556, 



Magnified 20 diameters. 



Amphitropous ovule of Mallow in different stages. 

 Fig. 556. Section. 



quently an excessive growth takes place at 

 one side of the coats of the ovule, so that 

 the chalaza is carried up and directed away 

 from the placenta, the micropyle being at 

 the same time turned down towards the 

 latter ; but as the growth is in the coats of 

 the ovule, the hilum remains at the base, 

 near where the micropyle arrives ; such an 

 ovule is termed anatropous (fig. 120, p. 131). 

 The hilum is then connected with the chalaza 

 by a ridge (a kind of adherent funiculus) 

 called the raphe. In other cases the form 

 becomes altered by the point of the ovule 

 turning down, and the entire structure beco- 



