278 ORGANOGRAPHY. 



ferent kinds of placentation, we proceed to consider the views 

 entertained as to their origin. It is generally believed that the 

 placenta is, in most eases at least, a cellular growth developed 

 from the confluent margins of the carpels (or, more strictly 

 speaking, from the confluent margins of the laminae of the 

 carpellary leaves), and bearing ovules upon its surface. In 

 some cases the placenta extends along the whole line of union 

 of the carpel, 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 

 like manner formed from the contiguous margins of each indi- 

 vidual 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. Before proceeding to 

 describe the nature of the free central placenta, it will be 

 necessary to conclude our notice of the above forms, as its de- 

 scription involves the discussion of a diflferent view of the origin 

 of the placenta. 



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 to the points of union of the margins of the 

 carpel, and hence would naturally be considered as formed from 

 them ; and secondly, we frequently find, that in monstrosities 

 or abnormal growths where the carpel is developed in a more 

 or less fiattened condition, a placenta bearing ovules is formed 

 upon each of its margins. The production of the ovules in 

 these cases may be considered as analogous to the formation 

 of buds on the margins of leaves, as in Bryophyllum calycioium 

 {fig. 187), already referred to. The formation of the placentas 

 from the margins of the carpels in axile and parietal placenta- 

 tion, may be considered, therefbre, as capable of being proved 

 by direct observation, and from analogy to what occurs in 

 certain ordinary leaves. 



We now pass to consider the origin of the free central pla- 

 centa. The theory formerly entertained was, 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 be- 

 tween them was soon destroyed ; and that from this cause, and 

 also from the great subsequent development of the placenta, the 

 septa ultimately became almost or quite broken up, so that the 



