206 
ORGANOGRAPHY. 
BOOK I. 
7W, Z, formed by the cohesion of the contiguous margins of the 
carpels a and b. 
5. A single carpel can have no true dissepiment, 
6. The dissepiment will alternate ivith the stigma : — for 
the stigma is the extremity of the midrib of the carpellary 
leaf, or of the dorsal suture of the carpel; and the sides of 
either of these (which form dissepiments) will be right and 
left of the stigma, or in the same position with regard to the 
latter organ as the sides of the lamina of a leaf to its apex. 
triangle 
a b c represent a 
of a three-celled ® 
Let the 
transverse section 
ovary, of which d, e, f are the dissepi- 
ments. The stigmas would occupy a 
position equal to that of the spaces 
5, 5, s, and would consequently be al- 
ternate with d, e, the dissepiments : 
they could not possibly be placed 
opposite d, e,f, upon any principle of 
structure with which we are acquainted. 
This law proves that neither the membrane which separates 
the two cells of a cruciferous siliqua, nor the vertical plate 
that divides the ovary of Astragalus into two equal portions, 
are dissepiments ; both are expansions of the placenta, or of 
some other part, in different degrees. 
All partitions whose position is at variance with the fore- 
going laws are spurious. Such spurious dissepiments are 
caused by many circumstances, the chief of which are the 
following: — they are caused by expansions of the placenta, 
as in Cruciferae, when they form a partition stretching from 
one side to the other of the fruit ; or they are mere dilatations 
of the lining of the pericarp, as in Cathartocarpus Fistula, in 
which they are horizontal ; or they are internal expansions of 
the dorsal or ventral suture, as in Amelanchier, Astragalus, 
and Thespesia, in which they are distinguishable from the 
dissepiments by not bearing the placentae, and by being 
opposite the stigma, or by projecting beyond the placentae; 
or, finally, they are caused by the sides of the ovary projecting 
into the cavity, uniting and forming many supernumerary 
cells, as in Diplophractum. 
