FUNDAMENTAL ORGANS. 



LnA 



times enormously hypertrophied, especially in Rosacece and Pomacece (figs. 707 to 711), 

 and bears on the top the stamens, petals, and calyx. 



Ovule. Botanists often apply this name to the undeveloped seed ; but, to be 

 precise, they ought to confine it to the unfertilized seed. 



To trace the development of the ovule, it must be examined long before the 

 bud opens : it then appears as a papilla on the placenta, called the nucleus (fig. 

 712) ; around the base of the nucleus (fig. 713) a circular ring is formed (s), which at 

 first grows at the same rate as itself, but, rising on its surface, it eventually 

 overtops and finally almost entirely envelops the nucleus ; but before this takes 

 place, a second circular ring is developed (fig. 714, p) outside the first (s), which 

 follows it in its growth, and ends by reaching and overtopping it ; the nucleus (N) is 

 hence enclosed in two sacs, whose mouths are contracted, and on a level with its top, 

 thus forming a little cylindrical or cup-shaped cavity, consisting of two superimposed 

 rings touching at all points of their circumference. The upper 

 opening, belonging to the outer coat, is named exostome (Ex) ; the 

 lower, belonging to the 

 inner coat, is named en- 

 dostome (End). The union 

 of the endostome and ex- 

 ostome constitutes the 

 micropyle, which always 

 corresponds to the top of 

 the nucleus. The outer 

 coat is called the primine (p), the inner the secundine (s) ; the nucleus (N) has also been 

 called the tercine ; terms which refer to their order of superposition from without 

 inwards, not that of development. The fuuicle (p) is inserted on the primine, and 

 its contained bundle of tracheae, after traversing the primine and secundine, expands 

 at the base of the nucleus into a swollen coloured cellular tissue, termed chalaza, 

 opposite to which there is almost always a corresponding swelling on the primine. 

 As the ovule, which is wholly composed of cellular tissue, grows, a cavity is formed 

 near the centre (fig. 715) of the nucleus, by the dilatation of one of its cells; this 

 cavity, which extends through the length of the nucleus, and adheres by its two 

 ends to the neighbouring cells, is the embryonic sac (S.E), or quintine. Its walls 

 shortly become lined with a mucilaginous cellular tissue, developed from the circum- 

 ference towards the centre, which fills the cavity of the sac ; this tissue, together 

 with that of the nucleus, constitutes the alimentary deposit destined for the embryo, 

 and is called albumen (perispermum). The ovule, thus organized before fertilization, 

 undergoes one of the three following changes : most frequently the embryonic sac 

 pushes away the nucleus on all sides, and its own parenchyma alone is developed, 

 when the albumen is more or less fleshy ; sometimes, on the contrary, the nucleus 

 presses upon the embryonic sac, and reduces it to a narrow tube, when the albumen 

 is farinaceous ; sometimes, again, the action is reciprocal, and two kinds of albumen 

 result, the white Nymphcea (figs. 610, 647) affords a remarkable instance of this. 

 For this reason Gaertner, comparing the ovule with a bird's egg, limited the term 



712. Mistleto. 

 Ovule (mag.). 



714. Polygonum. 

 Ovule (mag.). 



715. Polygonum. 

 Ovule cut vertically 

 (mag.). P, primine; S, 

 secundine ; N, nucleus ; 

 S.E, embryonic sac. 



