284 



ORGANOGRAPHY. 



placentation is called central, and the term axile is restricted to 

 the form of placentation Avhere the placenta is supposed to be a 

 prolongation of the axis. This will be afterwards alluded to. 



In a compound one -celled ovary there are two forms of 

 placentation, namely, the parietal, and the free central. It is 

 termed parietal, Avhen the ovules are attached to placentas 

 either placed directly on the wall of the ovary, as in Reseda 

 {fig. 606), and Cactus {fig. 616), or upon incomplete dissepi • 

 ments formed (as already noticed) by the partially infolded 

 carpels, as in the Orchis {Jig. 607) and Poppy {fig. 608). In 

 parietal placentation, the number of placentas corresponds to 

 the number of carpels of which the ovary is formed. Wiien the 

 placentas are not attached to the walls of the ovary, but are 

 situated in the centre of the cavity and perfectly unconnected 

 with those Avails, they fcn-m what is called a. free central placenta, 

 as in the Carvophvllaceai (^^r.?. 618 and 619), PrimulacetB {fig. 

 620\ &c. 



Fig. 617. 



Fig. 619. 



Fig. 617. Transverse section of the 

 young ovary of Lychnis, show- 

 ing five partitions i)ro(;ee(ling 

 from the walls of tlie ovary to 

 the placentas in tiie centre; these 

 partitions are destroyed by the 

 growth of tlie ovary, so tliat tlie 

 placentation is ultimately free. 



. Fi(i. 018. Vertical section of the Cerastiiim tiirsutum (Canjophnllacefe), 



o. Ovary, p. Free central placenta, f/. Ovules, s. Styles Fie/. t;i9. 



Transverse section of the same with the two portions separated, o. Ovary. 

 p. Placenta. </. Ovules, s. Styles. From Jussieu. 



Besides the regular kinds of placentation just described, it 

 sometimes happens that the ovules arc ])laccd more or less ir- 

 regularly in tiic cavity of the ovary. Thus, in the Flowering 

 Kusli {Butumns) {fig. 621) they cover the whole inner surface 

 of the carpels ; in tlic A[i/i)iphcea, they are attached all over the 

 dissepiments ; in Cabomba, they arise from the dorsal suture ; 

 and in Broomrapo {Orobdttche), from placentas placed within the 

 margins of tlic ventral suture. 



2. Origin of the. Placenta. — Having now described the differ- 

 ent kinds of i»laccntation. we proceed to consider tlie views 

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



