ORGANS OF REPRODUCTION. 



Fig. 620. Fig. 621, 



285 



Fig. 620. "Vertical section of tlie pistil of Cyclamen (.Primulace.ce). s. Sepals. 



j)l. Free central placenta, st. Style, sf/fir. Stigma Fi'j. 621. Vertical 



section of the flower of the Flowering Rush, showing the inner surface of 

 the carpels covered all over with ovules. 



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

 from the confluent marghis of the carpels, 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 de- 

 veloped by a single carpel ; in compound many-celled ovaries, 

 the placentas are in like manner formed from the contiguous 

 margins of each individual carpel of which it is composed ; 

 while in compoimd one-celled ovaries presenting parietal pla- 

 centation, 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 forais, as its description involves the discussion of a dif- 

 ferent 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 tlie 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 flattened condition, that 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 calycinum 

 {fig. 195), already referred to. The formation of the placentas 



