172 
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 carpellary 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 tlie 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 Boragineae, Labiatae, Umbelli- 
ferae, Stellatae, Compositae, and many others. There can be 
no doubt, however, that ail 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 are 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 
