MORPHOLOGY OF EEPEODUCTIVE ORGANS 201 



of the ovary, but are situated in the centre of the cavity and 

 perfectly unconnected with the wall, fchey form what is called a 

 free central placenta, a.a in the Caryophyllaceae (figs. 436, j)?, 

 and 437, jj) and the Primulacese (fig. 438, j^?). 



Besides the regular kinds of placentation just described, it 

 sometimes happens that the ovules are placed more or less 

 irregularly in the cavity of the ovary. Thus, in the Flowering 

 Rush (fig. 439), thej' cover the whole inner surface of the car- 

 pels except the midribs ; and the placentation is consequently 

 described as superficial. Other irregidarities also occur : in 

 Nymphcea, they are attached all over the dissepiments ; in ■ 

 Cabomba, they arise from the dorsal suture ; and in the Broom- 

 rapes (Orobanchc) , from placentas placed within the margins of 

 the ventral suture. 



b. Origin of the Placenta. — 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. 431, ^Z) ; in others it may be confined 

 to its base or apex. Each placenta is to be considered 

 as composed of two halves, one half being formed by each 

 margin of the carpel. In simple ovaries the placenta is 

 developed by a single carpel. In compound many-celled 

 ovaries the placentas are formed by the union at the centre 

 of the ovary of the inturned margins of the cai'pels ; whUe in 

 compound one-celled ovaries presenting parietal placentation, 

 each placenta is formed from the contiguous margins of two 

 carpels which do not meet in the centre. 



That the placentas are really developed in the above forms 

 of placentation from the margins of the carpels is proved 

 in various ways. In the first place, the placentas always 

 correspond in regular kinds of placentation to the lines of 

 union of the margins of the carpel or carpels, and hence may 

 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 flattened con- 

 dition, a. placenta bearing ovules is formed upon each of the 

 margins. 



The origin of the free central placenta is not always the same ; 

 there are two different types recognisable. In the first, this 

 also is a development from the margins of the carpels. The 

 carpels of which the compound ovary is formed originally meet 



