378 



DISSEPIMENTS. 



[BOOK i, 



Bryophyllum, Malaxis paludosa, and most other cases, they 

 usually spring from the edge of the leaf, they also arise from 

 its surface in ferns ; and in such cases as that of the Ornitho- 

 galum leaf mentioned by Turpin (fig. 19, p. 172) 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, 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 in ovaries, that are formed by the united 

 sides of cohering carpels, and which separate the inside into 

 cells, are called dissepiments or septa. In order to form an 

 exact idea of the structure of pistils, it is important to bear 

 in mind, not only that such is really their origin, but that 

 they cannot possibly have any other origin. 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 cir- 

 cumstance 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 



fig. 128. 



e a c ft 



f b d 



if a b, in, fig. 128., represents the side of one carpel, and c d, 

 that of another, the dissepiment a c b d, formed by this union, 



