190 Mr. B. Clarke on Relative Position ; 



In Omalanthus among Euphorbiacese; the tricarpous and dicai-- 

 pous ovaries are to be seen on the same plants and as tricarpous 

 ovaries have generally one carpel posterior, its disappearance 

 sufficiently accounts for such ovaries when reduced to two carpels 

 having them placed laterally. Two carpels right and left some- 

 times, however, arise from the suppression of two carpels of an 

 ovary consisting of four, of which Caprifoliacese and Cruciferae 

 contain examples. 



2. When the two carpels are anterior and posterior. In Hout- 

 tuynia cor data the ovary is tricarpous, one carpel being posterior 

 and two lateral ; but frequently it becomes dicarpous ; in these 

 instances a lateral carpel generally disappears, first leaving the 

 other two anterior and posterior, as afterwards more particularly 

 adverted to. 



In Agrimonia, although the ovary is apocarpous, the thi'ce 

 carpels are almost uniformly two of them lateral and one pos- 

 terior, and in abortion a lateral carpel first becomes rudimentary 

 or ceases to be developed, the remaining two being left (with but 

 very few exceptions) anterior and posterior ; thus corresponding 

 in position with Spiraa when dicarpous. 



In Reseda luteola, which frequently has dicarpous ovaries, the 

 two carpels are anterior and posterior, or less frequently oblique 

 from the absence of a lateral carpel, as the three carpels of 

 Reseda have the same relation to the axis as in Houttuynia and 

 Agrimonia. 



These and other analogous examples make it evident that an 

 ovary consisting of two carpels anterior and posterior generally 

 results from a tricarpous ovary, one of the lateral carpels of 

 which is not developed, the other lateral carpel having become 

 in consequence anterior, having been removed from its position 

 to be opposite the posterior carpel. Another instance as occur- 

 ring in Heracleum is mentioned by Mr. Ralph (vide Proceedings 

 of Linn. Soc. vol. i. p. 284). 



3. When the two carpels have an oblique relation to the axis. 

 Although perhaps in no instance are the carpels when two always 

 oblique, yet the oblique position frequently occurs, both in plants 

 in which the carpels are generally anterior and posterior, and in 

 those in which they are as predominantly right and left. This 

 oblique position probably arises from the lateral carpel of a tri- 

 carpous ovary not becoming anterior when the other lateral car- 

 pel has disappeared, but remaining nearly in its original position, 

 in consequence of which the posterior carpel is somewhat dis- 

 placed, becoming obliquely posterior; and thus antei-ior and pos- 

 terior and oblique may be regarded as one and the same cha- 

 racter, and as a general rule this may prove available, but two 

 lateral carpels do undoubtedly sometimes become oblique. 



