560 ON THE RELATIVE POSITION OF 



arrangements the adaptation to the performance of function 

 is equally manifest. 



If the correctness of these observations be admitted, it 

 follows that characters dependent on the various mocUHca- 

 tions of stigmata are of less value, both in a systematic point 

 of view as determining the limits of families, and theoreti- 

 cally in ascertaining the true composition of organs, than 

 those derived from the analogous differences in the ovaria 

 or placentae. 



In those cases in which the nature of the composition of 

 the ovarium is doubtful, it may, in the first place, be re- 

 marked, that wherever in the compound unilocular pistillum 

 the placentae are double or two-lobed, it is more probable 

 that such placentae are derived from two adjoining carpels, 

 and are consequently marginal or submarginal, than that 

 they occupy the disc of one and the same carpel ; this being 

 entirely the appearance in many cases where the marginal 

 origin of placentas is admitted ; while in the greater part of 

 those in which the disc is known to be ovuliferous, the ovula 

 are never collected in two distinct masses, being generally 

 scattered equally over the surface. 



But the double placentae are manifest in Orchidece, the 

 principal family in which ]Mr. Lindley considers the ovula 

 as occupying the disc and not the margins. In this family 

 also the alternation of stigmata with placentae is that rela- 

 tion which is most usual in compound unilocular ovaria, 

 where the apparent number of stigmata and ])lacentae is 

 equal ; and that in Orclddece each apparent stigma is formed 

 bv the confluence of the two stionnata of one and the same 

 carpel, is proved by tracing to their origins their vascular 

 cords, Avhich are found to coalesce with those of the three 

 outer foliola of the perianthium. 



This view of the composition of the ovarium in Orcltldcce 

 is confirmed by finding that it agrees with the ordinary 

 arrangement in monocotyledonous plants ; namely, the 

 opposition of the double parietal placentae to the three inner 

 divisions of perianthium^ while in Aposfasia the three pla- 

 centae of the trilocular ovarium are opposite to the three outer 



1 Deuham, Trav. in Afr. Append, p. 243. {AnU, p. 300.) 



