226 
ORGANOGRAPHY. 
BOOK I. 
Valvular dehiscence, which is by far the most common 
mode by which pericarps open, must not be confounded with 
either rupturing or solubility , — irregular and unusual con- 
trivances of nature for facilitating the dispersion of seeds. In 
valvular dehiscence the openings have a certain reference to 
the cells, as has been already shown ; but neither rupturing 
nor solubility bear any distinct relation to the cells. Rup- 
turing consists in a spontaneous contraction of a portion of 
the pericarp, by which its texture is broken through, and 
holes formed, as in Antirrhinum and Campanula. Solubility 
arises from the presence of certain transverse contractions of 
a one-celled pericarp, through which it finally separates into 
several closed portions, as in Ornithopus. 
For the nature of the placenta and umbilical cord see the 
observations under ovary. These parts, which are mere 
modifications of each other, essentially appertain to the peri- 
carp, in which the former often acquires a spongy dilated 
substance, occasionally dividing the cells by spurious dissepi- 
ments, and often giving to the fruit an appearance much at 
variance with its true nature. In some seeds, as Euonymus 
europaeus, it becomes exceedingly dilated around each seed, 
forming an additional envelope, called aril. The true charac- 
ter of this organ was unknown till it was settled by Richard : 
before his time the term was applied, not only in its true sense 
to an enlargement of the placenta, but also to the endocarp of 
certain Rubiacese and Rutaceae, to the seed-coat of Jasminum, 
Orchideae, and others, and even to the perianth of Carex. A 
very remarkable instance of the aril is to be found in the 
nutmeg, in which it forms the part called the mace surround- 
ing the seed. It is never developed until after the fertilis- 
ation of the ovule. 
Having thus explained the structure of the pericarp, it is 
in the next place necessary to enquire into the nature of its 
modifications, which in systematic botany are of considerable 
importance. It is, on the one hand, very much to be regretted 
that the terms employed in this department of the science, 
which is that of Carpology, have been often used so vaguely 
as to have no exact meaning ; while, on the other hand, they 
have been so exceedingly multiplied by various writers, that 
