ESSENTIAL ORGANS — THE OVULE. 



253 



Fig. 460. 



Fig. 451. 



{funis, a cord), umbilical cord, ' or podosperm (orous, a foot, and 

 enti^fia, a seed). This cord sometimes becomes much elongated after 

 fertilisation. The placenta is sometimes called the trophospertn (r^ifio, 

 I nourish). The part by which the ovule is attached to the placenta 

 or cord is its base or hilum, the opposite extremity being its apex. 

 The latter is frequently turned round in such a way as to approach 

 the base. The ovule is sometimes imbedded in the placenta. 



In its simplest form, as in the Mistleto, the ovule appears as a 

 small cellular projection. The cells multiply until they assume a 

 more or less enlarged ovate form, constituting what has been called 

 the nucleus (figs. 450, 451 n), or central cellular m'ass of the ovule. 

 The ovular nucleus^alters in the progress of growth so as to be prepared 

 for the development of the embryo 

 plant in its interior. At the apex of 

 the cellular nucleus, an absorption or 

 obliteration of cells takes place, by 

 which a hollow cavity is formed (fig. 

 451 c), which in some plants becomes 

 lined by a thin layer of cells or epithe- 

 lium (p. 236), whilst in others the cells 

 of the nucleus alone form its walls. 

 This cavity is the embryo-sac, and contains amnios or mucilaginous 

 fluid, in which, after fertilisation has been completed, the embryo 

 plant is formed, being attached to the apex of the sac by a thread- 

 like cellular process called the suspensor. 



The nucleus (fig. 457 n) may remain naked, and alone form the 

 ovule, as in the Mistleto, and a few other plants; but in most 

 plants it becomes surrounded by certain coverings during its de- 

 velopment. These appear first in the form of cellular rings at the 

 base of the nucleus, which gradually 

 spread over its surface. In some 

 cases only one covering is formed, 

 as in Oompositse, Oampanulacese, 

 Walnut, etc. Thus, in the latter 

 (fig. 452), 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. 453), leaving only 

 w. opening at the apex. In other instances (fig. 454), the nucleus, n. 



Fig. 460. Ovule of the Mistleto entire. Fig. 451. Ovule of Mistleto out to show the 

 embryo-sac, c, and the whole of the rest of the mass, n, compdsed of uniform tissue, and 

 forming a nucleus without integuments. Fig. 452. Ovule of Juglans regia, the Wahiut. 

 t. Simple integument, n. Nucleus, the base of which only is covered with integument at 

 the early period of development. Fig. 453. The same ovule more advanced, in which the 

 nucleus is nearly completely covered. 



Fig. 462. 



Fig. 453.1 



