204 
ORGANOGRAPHY. 
BOOK I. 
position of the placentas with regard to the margins of the 
carpel is reducible to no certain rule, but depends upon 
specific organization. Consequently we shall no longer be 
unable to account for the unusual situation of the placentae 
opposite the stigma, in Papaver (as M. Kunth has lately 
noticed), in Parnassia, or elsewhere. 
We ought not indeed to be surprised at coming to this 
result; for if the ovules are, as botanists generally believe 
them to be, a modification of buds, then the uncertainty in 
the position of the placentary lines v/ill only be conformable 
to the uncertainty in the origin of buds from leaves. If in 
Bryophyllum, Mai axis paludosa, and most other cases, they 
usually spring from the edge of the leaf, they also arise from 
its surface in ferns ; and in the famous case of the Ornitho- 
galum leaf mentioned by Turpin, they were found issuing 
indiscriminately from all parts of its face. 
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, con- 
sequently, 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 in ovaries, that are formed by the united 
sides of cohering carpels, and which separate the inside into 
cells, are called dissepiments or septa. It is important 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, Pentstemon, and a 
multitude of other plants. The consideration of this circum- 
stance leads to certain laws which cannot be subject to ex- 
ception, but which are of great importance; the principal of 
which are these ; — 
1. All dissepiments are vertical and never horizontal. — For 
if a b, in Ji^. 128., represents the side of one carpel, and c d, that 
