ESSENTIAL ORGANS. THE OVULE. 



225 



or hilum, the opposite extremity being its apex. The latter is fre- 

 quently turned round in such a way as to approach the base. The 

 ovule is sometimes imbedded in the placenta. 



463. In its simplest form, as in the Misletoe, the ovule presents itself 

 as a small cellular projection, which enlarges, assumes an ovoid form 

 (fig. 414), and ultimately becomes 



hollowed at the apex (fig. 415 c). 

 The cavity thus produced is sur- 

 rounded by a mass of cells called the 

 nucleus, n, and is destined to contain 

 the embryo plant after the process of 

 fertilization has been completed. In 

 this embryonal cavity the young plant 

 is suspended by a thread-like cellular 

 process called suspensor, attached to the summit of the nucleus. This 

 cavity in some plants is surrounded by the cells of the nucleus ; but, 

 in other cases, it becomes lined with a regular epithelial (^[ 431), or thin 

 cellular covering, and constitutes the embryo-sac, which is produced 

 before fecundation, and contains amnios or mucilaginous matter in 

 which the embryo is formed. 



464. The nucleus (fig. 421 n) may remain naked, and form the ovule, 

 as in the Misletoe, Veronica hederifolia, Asclepias, &c.; but in most 

 plants it becomes surrounded by certain coverings during the progress 

 of development. These appear first in the form of one or more cellular 

 rings at the base, which gradually spread over the surface. In some 

 cases only one covering is formed, as in Composite, Campanulaceas, 

 Walnut &c. Thus, in the latter (fig. 416), the nucleus, n, is covered 

 by a single envelope, t, which, in the first instance, extends over the 

 base, and then spreads over the 



whole surface (fig. 417), leaving only 

 an opening at the apex. In other 

 instances (fig. 418), the nucleus, n, 

 besides the single covering (fig. 418, 

 2, fe'), has another developed sub- 

 sequently (fig. 418, 3, te), which 

 gradually extends over the first, 

 and ultimately covers it completely. 

 There are thus two integuments, an 



outer and an inner the latter, according to Schleiden, being first pro- 

 duced. Mirbel considers the outer as the first formed, and hence has 

 called it primine, te, while the inner is denominated secundine, ti. The 



Fig. 414. Ovule of the Misletoe entire. 



Fig. 415. Ovule of Misletoe cut to show the embryo-sac, c. and the whole of the rest of the 

 mass, n, composed of uniform tissue, and forming a nucleus without integuments. 



Fig. 416 Ovule of Juglans regia, the Walnut, t, Simple integument. , Nucleus, the base of 

 which only is covered with integument at the early period of development. 



Fig 417. The same ovule more advanced, in which the nucleus is nearly completely covered. 



