262 



MORPHOLOGY 



becomes curved, the micropyle being directed thus towards the surface 

 of origin; such ovules are called campylotropous (fig. 583), and they are 

 much less common than the other kinds. Far the must common kind of 

 ovule among angiosperms is one which develops a stalk (funiculus) that 

 becomes curved at the apex, so that the body of the ovule lies against it, 

 and although the axis of the body is straight, the micropyle is directed 

 towards the surface of origin; such ovules are called anatropous (in- 

 verted), the funiculus appearing as a ridge along one side of the body of 

 the ovule (figs. 581, 584). The advantage 

 of the anatropous ovule may be recognized 

 when it is remembered that the pollen tube 

 is advancing along the wall of the ovary, 

 and the micropyles are thus brought near the 

 wall. 



Development. — The megasporangium 

 (really the nucellus) is eusporangiate in its 

 development, resembling the microsporan- 

 gium at every stage. There is usually a 

 single hypodermal initial cell, which is soon 

 recognized among the other hypodermal 

 cells by its larger size and the different ap- 

 pearance of its contents (fig. 585). Some- 

 times there are two or more of these initial 

 cells, as is the usual case in microsporangia. 

 The large hypodermal initial divides by a 

 periclinal wall into two cells, the outer cell 

 being the primary wall cell, the inner one 

 being the primary sporogenous cell (fig. 586). 

 The wall cell may not divide (fig. 587), or 

 there may be one or more divisions (fig. 588), 

 or in some cases there may be several wall layers developed, as in 

 microsporangia. The primary sporogenous cell does not divide and 

 form more sporogenous cells, and therefore it is the megaspore mother 

 cell. This means that when it divides, a tetrad is formed by two 

 successive divisions, which are the reduction divisions. The tetrad of 

 megaspores is almost always a linear row (fig. 587), which is an ex- 

 ceptional arrangement among microspores. It is very seldom that 

 more than one of the megaspores matures, and that one is almost 

 invariably the innermost one of the row, that is, the one farthest from 



Figs. 585, 586. — Develop- 

 ment of megasporangium of 

 Salix: 585, single hypodermal 

 initial; 586, division of initial 

 into primary wall cell (outer 

 shaded one) and primary spo- 

 rogenous cell. — After Cham- 

 berlain. 



