254 ESSENTIAL ORGANS — THE OVULE. 



besides the single covering (fig. 454, 2, ti), has another developed sub- 

 sequently (fig. 454, 3, U), which gradually extends over that first 

 formed, and ultimately covers it completely, except at the opening at 

 the apex. There are thus two integuments to the nucleus, an outer 

 and an inner, called respectively .pnOTme, te, and secundine, ti. The 

 name tercine has been given to the cells of the nucleus which surround 

 the embryo-sac (fig. 451 n). These names aie applied to the coverings 

 of the ovule without reference to their order of development. At the 



apex of the ovule the primine 

 '®' ■ ^'^ and secundine leave an open- 



ing termed the foramen or 

 micropyle (//,ixghg, small, and 

 •s-iiXjj, a gate). This foramen 

 extends through both coats, 

 the opening in the primine 

 (fig. 454, 3, ex), being the exo- 

 siome(£^iia, outside, and sro/j^a, 

 mouth, that in the secundine 

 (fig. 454, 3, ed), being the endostome (sv^oi/, within). The micro- 

 pyle indicates the organic apex of the ovule, while the part united directly 

 or by the funiculus to the placenta is the base or hilum. The name mi- 

 cropyle is sometimes restricted to the foramen in the perfect seed. The 

 length of the canal of the foramen depends on the development of the 

 nucleus, as well as on the thickness of the integuments. The embryo- 

 sac is sometimes prolonged beyond the apex of the nucleus, as noticed 

 by Meyen in Phaseolus and Alsine media, and by Griffith in Santalum 

 album and Loranthus. Some authors, as Mirbel, considering the 

 ovule in reference to the embryo, speak of five coverings of the latter — 

 viz. 1, primine ; 2, secundine ; 3, tercine, or the covering of the nucleus 

 lining the secundine ; 4, guartine, a temporary cellular layer, which is 

 occasionally formed at an after period in the form of perisperm around 

 5, quintine, or the embryo-sac. By most botanists the nucleus and 

 sac, with its two integuments (primine and secundine), are mentioned 

 as the ordinary structure of the ovule. Occasionally, as in Mistleto, 

 there are two or three embryo-sacs formed. In Veronica and Euphrasia 

 the neck of the embryo-sac becomes elongated and swollen, and from 

 it are developed certain cellular or filamentous appendages, which are 

 probably connected with the nutrition of the embryo. 



All these parts are originally cellular. The nucleus and integu- 



Fig. 454. Ovule of Polygonum cymosum at various ages, n, Nucleus, te. The outer in- 

 tegument or primine. ti, The inner integument or secundine, ex, Exostome or opening in 

 the primine, ed, Endostome or opening in the secundine. 1, Ovule in the early state, when 

 the nucleus is still naked. 2, Ovule in second stage, when the nucleus is covered at its 

 tase hy the internal integument or secundine only. 3, Ovule in the third stage, when the 

 two integuments, primine and secundine, form a double covering, at the apex of which the 

 nucleus still appears. 



