17'2 ORGANOGRAPHY. BOOK I. 



I have stated that the placenta, however simple it may ap- 

 pear to be, is really the result of the union of two united mar- 

 gins of a carpellaiy leaf : it is, therefore, essentially double ; 

 and, accordingly, we find that in polyspermous ovaries the 

 ovules are almost always arranged in two rows, as in the Pea 

 and Bean, the Quince, the Paeony, &c. ; nevertheless there 

 are instances in which the placentae occupy a considerable 

 portion of the wall of the ovary, and bear the ovules in a 

 great many rows, but in no certain order, as in Nymphaea ; 

 and, on the other hand, some plants have the placentae so 

 little developed, that not more than one ovule is generated 

 between the two placentae, as in Boraginese, Labiatae, Umbelli- 

 ferae, Stellatae, Compositae, and many others. There can be 

 no doubt, however, that all the latter cases are mere instances 

 of suppressed structure, in consequence of the general incom- 

 pleteness of developement. 



When two leaves are developed upon a stem, they are 

 always opposite, and never side by side. As carpels are 

 modified leaves, they necessarily^ obey this law; and, conse- 

 quently, when a pair of carpels forms a bilocular ovarium, the 

 separation of the two cells is directly across the axis of the 

 flower. 



The partitions that ai*e formed in ovaries, by the united 

 sides of cohering carpels, and which separate the inside into 

 cells, are called dissepiments or septa. It is extremely im- 

 portant to bear in mind, not only that such is really their 

 origin, but that they cannot possibly have any other origin, 

 in order to form an exact idea of the structure of pistils. 

 Now, as each dissepiment is thus formed of two united sides, 

 it necessarily consists of two plates, which are, in the ovary 

 state, often so completely united, that their double origin is 

 undiscoverable, but which frequently separate in the ripe 

 pericarp. This happens in Rhododendron, Euphorbia, Pent- 

 stemon, and a multitude of other plants. The consideration 

 of this circumstance leads to certain laws which cannot be 

 subject to exception, but which are of great importance ; the 

 principal of which are these : — 



1 . All dissepiments are vertical and never horizontal. — For 



