Stir/ma and Parietal Placenta in Plants. 39 



pound but unilocular ovarium, while the placentae of the ad- 

 joining carpels are united, the stigmata of each carpel are 

 generally confluent. But this rule admits of exceptions, as 

 in Parnassia, in many Cruciferce, and in Papaveracece : in all 

 these cases the stigmata as well as placentae of the adjoining 

 carpels are confluent, a structure satisfactorily proved in Cru- 

 ciferce by several cases of monstrosity, in which the stamina 

 are transformed into pistilla ; and in Papaveracece by a series 

 of modifications cf structure as well as by a like transforma- 

 tion of stamina. 



A similar confluence of stigmata in the compound multi- 

 locular pericarpium is of much rarer occurrence ; it is found, 

 however, in the majority of Iridece, in which the three stig- 

 mata alternate with the cells, and consequently with the pla- 

 centae of the trilocular ovarium. That this is the correct 

 view of the composition of the stigmata in Iridece is at least 

 probable from their occasional deep division, and more parti- 

 cularly still from the bifid petal-like styles or stigmata which 

 are opposite to the cells of the ovarium in other genera of the 

 same family, as in Iris and Moraa. In both these arrange- 

 ments the adaptation to the performance of function is equally 

 manifest. 



If the correctness of these observations be admitted, it fol- 

 lows that characters dependent on the various modifications 

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

 view as determining the limits of families, and theoretically in 

 ascertaining the true composition of organs, than those de- 

 rived 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 remarked, 

 that wherever in the compound unilocular pistillum the pla- 

 centae are double or two-lobed, it is more probable that such 

 placentas are derived from two adjoining carpels, and are con- 

 sequently marginal or submarginal, than that they occupy the 

 disc of one and the same carpel : this being entirely the ap- 

 pearance in many cases where the marginal origin of placentae 

 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 prin- 

 cipal family in which Mr. Lindley considers the ovula as oc- 

 cupying the disc and not the margins. In this family also 

 the alternation of stigmata with placentae is that relation which 

 is most usual in compound unilocular ovaria, where the ap- 

 parent number of stigmata and placentae is equal ; and that 



