556
ON THE RELATIVE POSITION OF
Secondly. Where the production of ovula is limited tothe external angle of the cell or axis of the leaf supposedto form the carpel.
A case of this kind is found in a portion of one of thosefamilies in which the whole surface is generally ovuliferous,namely, in Hydropeltidea, which 1 have always regarded asmerely a section of Nymphceaceee j 1 and from the nature ofthese differences in placentation, which are more apparentthan real, an argument might even be adduced in favour ofthat opinion.
A placenta apparently limited to the outer angle of the cellalso occurs in the greater number of species of Mesembry-anthemum. As this structure, however, is certainly notwithout exception in that very natural genus, several species,among which are Mesembrycinthemum crystallinum, cordi-folium , papulosum and nodiflorum, having the placenta con-fined to the internal angle of the cell or margins of thecarpel; and as in some of those species in which the outerangle 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 perhapsbe explained by assuming cohesion of the inflected portionof the carpel with the wall of the cell;—an hypothesis, insome degree supported by the fact, that in several speciesthe termination of the assumed inflected portion is free andnot ovuliferous.
But whatever opinion may be adopted as to the relationof this seemingly anomalous to the ordinary structure, itcannot, as M. Fenzl proposes, 2 be employed as the essentialcharacter of a distinct natural family limited to the Linrueangenus Mesernbryanthemum.
The placenta then of a simple ovarium in its usual state,according to this view, is necessarily double; though by thecomplete suppression of ovula in one of its two componentparts, and their diminished production in the other, theovarium is not unfrequently reduced to a single ovulurn.That such is the origin of the single ovulum is at least
1 Gen. Rem. in Flinders's Voj\ vol. ii, Append, p, 598. (Anti p. 74.)