556 ON THE RELATIVE POSfTION OF
Secondlv. Where the production of ovula is limited to the external angle of the cell or axis of the leaf supposed to form the carpel.
A case of this kind is found in a portion of one of those families in which the whole surface is generally ovuliferous, namely, in IlydropeUidece., wdiich 1 have always regarded as merely a section of Nyinphceacece •} and from the nature of these differences in placentation, wdiich are more apparent than real;, an argument might even be adduced in favour of that opinion.
A placenta apparently limited to the outer angle of the cell also occurs in the greater number of species of Mesemhry- anfJiemiim. As this structure, however, is certainly not wdthout exception in that very natural genus, several species, among which are Mcsemhryanfhernum crystaUinum, cordi- folium, pajmlosum and nodiflorum, having the placenta con- fined to the internal angle of the cell or margins of the carpel ; and as in some of those species in which the outer angle is placentiferous, the production of ovula is not con- fined to it, but extends to the lower half of the inner angle ; — this apparent deviation from ordinary structure may perhaps be explained by assuming cohesion of the inflected portion of the carpel with the wall of the cell ; — an hypothesis, in some degree supported by the fact, that in several species the termination of the assumed inflected portion is free and not ovuliferous.
But whatever opinion may be adopted as to the relation of this seemingly anomalous to the ordinary structure, it cannot, as M. Fenzl proposes," be employed as the essential character of a distinct natural family limited to the Linnaean genus Mesembryantherimm.
The placenta then of a simple ovarium in its usual state, according to this view, is necessarily double ; though by the complete suppression of ovula in one of its two component parts, and their diminished production in the other, the ovarium is not unfrequently reduced to a single ovulum. That such is the origin of the sinde ovulum is at least
^ Gen. Pvem. in Fiinders's Voy. vol. ii. Append, p. 59$. {Ante j}, 74.) 2 Aimal. des Wieii, Mus. vul.'i, p. 349,
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