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284 ORIGIN OF THE PLACENTA. 
Cabomba, they arise from the dorsal suture ; and in the Broom- 
rapes (Orobanche), from placentas placed within the margins of 
the ventral suture. 
2. Origin of the Placenta.—It is generally believed that the 
placenta is, in most cases at least, a cellular growth developed 
from the confluent margins of the carpels, or, more strictly 
speaking, from the confluent margins of the blades of the 
carpellary leaves. In some cases the placenta extends along 
the whole line of union of the carpel (fig. 628, pl), or it may be 
confined to its base or apex. Each placenta is therefore to be 
considered as composed of two halves, one half being formed by 
each margin of the carpel. Thus in simple ovaries the placenta 
is developed by a single carpel ; in compound many-celled ovaries 
the placentas are in hke manner formed from the contiguous 
margins of each individual carpel of which it is composed ; while 
in compound one-celled ovaries presenting parietal placentation, 
each placenta is formed from the contiguous margins of two 
carpels, and is hence produced by two adjoining carpels. 
That the placentas are really developed in the above forms 
of placentation from the margins of the carpels seems to be 
proved in various ways. Thus, in the first place, the placentas 
always correspond in regular kinds of placentation to the points 
of union of the margins of the carpel or carpels, and hence 
would naturally be considered as formed from them; and 
secondly, we frequently find, that in monstrosities or abnor- 
mal growths where the carpel is developed in a more or less 
flattened condition, a placenta bearing ovules is formed upon 
each of its margins. The productions of the ovules in these 
cases may be considered as analogous to the formation of 
buds on the margins of leaves, as in Bryophyllum calyeonum 
(fig. 217), and Malaxis paludosa (fig. 218), already referred to. 
The formation of the placentas from the margins of the carpels 
in axile and parietal placentation may be considered, therefore, 
as capable of being proved by direct observation, and from 
analogy to what occurs in certain ordinary leaves. 
But in reference to the origin of the free central placenta 
two different views are entertained. Thus it was formerly 
supposed that this also was a development from the margins of 
the carpels. It was thought that the carpels of which the 
compound ovary was formed originally met in the centre and 
developed placentas from their margins in the same manner as 
in ordinary axile placentation, but that subsequently the walls 
of the ovary grew more rapidly than the dissepiments, so that 
the connexion between them was soon destroyed ; and that 
from this cause, and also from the great subsequent develop- 
ment of the placenta, the septa ultimately became almost or 
quite broken up, so that the placenta was left free in the cavity 
of the ovary. ‘This theory is strengthened by the fact, that in 
several of the Caryophyllaceze we often find dissepiments in the 
