158 BOTANY 
To understand the structure of the ovary it will be 
necessary to revert once more to general principles. 
Already we have seen how ferns bear their repro- 
ductive organs along the edges of their leaves 
(Fig. 102). We may eonsider earpels as similar 
leaves which have folded in such a way as to bring 
their margins together and enclose the reproductive 
organs. In the pea, there is only one leaf, and, by 
cutting out an oblong piece of paper and marking the 
ovules along the edge, a rough model of this may be 
obtained. Now fold the two edges together, and we 
have a pod with the ovules attached along either side 
of the line of union. This line of union forms the 
ventral suture while the fold down the back which 
indicates the position of the midrib of the original 
leaf is called the dorsal suture. The pod shown in 
Fig. 103 is split along the dorsal suture. The 
placentation of the ovules, when thus attached to the 
inside of an exterior wall, is said to be parietal 
(L. paries a wall.) When, however, a single ovule 
rises from the base of the ovary, as in each carpel of 
the buttercup, the placentation is basal. Let us next 
consider ovaries that are formed from more than one 
carpel (Fig. 104). In the violet, three carpels are 
united by their edges to form a single chamber. 
The placentw, or parts to which the ovules are 
attached, are at the three junctions of the earpellary 
leaves. Thus there are three groups of ovules 
springing from the interior of the wall and the 
placentation is again parietal. In the lily, however, 
the edges of the three leaves fold right in till they 
meet at a point, and at the same time grow together to 
form a single body. In this way a round _ three- 
chambered structure is formed with the placente all 
massed together along the central axis. Each chamber 
or cell has at its inner corner a mags of ovules derived 
from the two edges of a carpellary leaf. This form of 
