Dr. Giraud's Contributions to f^egetahle Embryology. 163 



membrane {primine ?), at the apex of which is presented the exostome, or mi- 

 cropyle (fig. \,b.), opening close by, and to the outside of the umbilicus: so 

 that the direction of the nucleus is ex-actly parallel with that of the axis of the 

 pistil. The conducting tissue of the style may be traced between the colu- 

 mella (fig. l,c.) and that prolongation of the carpellary leaf which forms the 

 style {d.), into the carpellary cavity, as far as the exostome, with which it is 

 brought in contact by the anatropous development of the ovule. The vessels 

 which proceed along the placenta to form the raphe, are spiral vessels and 

 annular ducts ; and at the point at which they make a turn downwards to- 

 wards the chalaza, many of them end in closed extremities (e.), while the 

 vascular structure of the raphe usually terminates in a single vessel. These 

 vessels, together with an analogous set which run along the dorsum of the 

 carpel, proceed from a larger bundle of vessels, which in the receptacle bifur- 

 cates into these two sets. 



Second Period. — During the expansion of the bud, before the dehiscence 

 of the anther, and therefore before impregnation, a small elliptical cavity 

 (fig. 2,y.) appears near the apex of the nucleus, having a delicate lining mem- 

 brane formed by the walls of the surrounding cells. This cavity is the em- 

 bryo-sac (" sac embryonnaire," Brongniart and F. G. F. Meyen ; " membrana 

 amnii" Malpighi ; " quintine," Mirbel). From the exostome a minute canal 

 (g.) may be traced in the apex of the nucleus, leading to the embryo-sac. The 

 apex of the embryo-sac incloses, at this period, a quantity of organizable mu- 

 cilage, containing many minute bodies having the appearance and character 

 of cytoblasts (Schleiden) (A.). 



' Third Period. — The apex of the nucleus, and of its tegumentary membrane, 

 is now inclined and approximated towards the axis of the pistil. The embryo- 

 sac is much enlarged and lengthened ; its mucilage has disappeared ; and in 

 its place there is formed an elongated diaphanous utricle (fig. 3, h.) {primary 

 utricle ; " ufricule primordiale" Mirbel ; " v^sicule embryonnaire," F. G. F. 

 Meyen ; " Vextremitd antdrieure du boyau pollinique,'' Schleiden) containing a 

 quantity of globular matter (" globulo-cellular cambium" Mirbel ; " cyto- 

 blasts^' Schleiden). This primary utricle is developed wholly within the em- 

 bryo-sac, from which it can be clearly seen to be distinct. 



Fourth Period (after impregnation has occurred). — The pollen tubes do not 



y 2 



