136 



ANATOMY. 



71fi. Polyiroiinm. 

 Fertilized ovule, cut 

 vertically (mag.). 



stapes of development 

 in the ovule (inag ). 



albumen (white of egg) to the tissue developed within the nucleus (fig. 610, N), and 

 gave that ofvitellus (yolk of egg) to the tissue developed in the embryonic sac (S.E). 

 Fertilization is announced by the appearance of a body (fig. 716) destined to 

 form the embryo, suspended from or near the top of the embryonic sac (s.e). At 

 first it consists of a vesicle (v.e), named embryonic vesicle, filled with .a granular mat- 

 ter, in which is formed first one cell, then others, 

 each of which bears a wjtoblast on its wall. The 

 upper and slender portion of this vesicle (fig. 717) is 

 the suspensor ; in the lower and swollen portion the 

 embryo is developed ; the vesicle and its suspensor 

 soon disappear, when the embryo develops, according 



x 1-j.i 'j. xii j'-LlJ 717. Dicotyledonous rni- 



to whether it is monocotyledonous or dicotyledonous, i,ry... in different 

 as we have already shown, and increases within the 

 cavity of the ovule, which it invades by absorbing 

 the albumen. If the albumen has solidified before the growth of the embryo, 

 the latter remains small and takes up less room ; and the absorption of the 

 albumen is then delayed till the period of germination. The ovule is not 

 always provided with two coats ; sometimes the inner coat alone (secundine) is 

 developed (Walnut, fig. 713) ; in others the nucleus remains naked (Santalacca-, 

 Mistleto, fig. 712). It is important to understand the changes the ovule may undergo 

 before fertilization ; changes due to unequal development altering the relative posi- 

 tions of its different parts. In theory, the hilum and chalaza correspond, and occupy 

 the base of the ovule, the micropyle being at the top or opposite end. If the 

 ovule develops uniformly, the arrangement is not disturbed, and the ovule is straight 

 or orthotropous (ov. orthotropum, fig. 716), and the embryo will also be straight. In this 

 case the position of the radicle answers to that of the micropyle, i.e. opposite the 

 hilum and chalaza, and the embryo is said to be antitropous (ov. antitropus, Nettle, 

 fig. 578). 



When the ovule develops unequally, one of two things may happen: l,the chalaza 

 (Ch, fig. 718) may be removed from the hilum towards the position occupied by the 

 top of the"] ovule ; which top, by a reverse movement, may be turned towards the 

 hilum ; the axis of the ovule thus making a half turn upon itself, like a compass- 

 needle turning from the north to the south pole. In this case the 

 hilum not having been displaced, the vascular bundle which connects 



718. Dandelion. 



Anntropous ovule 



cub vertically (mag.). 



719. 720. 721. 722. 



Chelidonium. Anatropous ovule in different stages of development (mag.). 



723. Vertical section 

 of fig. 722. 



it with the chalaza is forced to follow the latter in its revolution, and form a more 

 or less projecting cord (E) in the thickness of the primine, named the raphe ; the 

 ovule is then reversed or anatropous (ov. anatropum, figs. 719 to 723). Here the embryo 



