liOTAMCAL SUBJECTS. 



269 



Fig. 109. Sorus of Trichomanes radicans (Hooker's " Brit. Ferns," 



pi. 42). 



a placenta or sorus, its prolongation as a bristle having acquired 

 for it the common name of Rooting Bristle fern, so that there is 

 good reason for considering the free central placenta as the 

 termination of a branch axis, as some botanists have considered it. 

 In connexion with the idea of buds and ovules being the 

 homologues of sporangia, in the " Gard. Cliron." of 12 January 

 1884, p. 57, occurs an interesting drawing of Scolopendrium 

 vulgare, var. cristatum (O'Kelly). It has a sagittate and cristate 

 frond. It is studded on its upper surface with buds, on the line of 

 sori, which normally would occupy the under surface. These 

 bulbils are simply buds on the veins, the latter being nothing but 

 branchlets of the midrib all fasciated into a frond. In this 

 case, however, instead of sporangia we have the veins breaking 

 out into buds, the frondlets of each bud being probably converted 

 sporangia. 



{d.) There is yet another form of placentation which, perhaj^s, 

 may not have received sufficient attention from botanists. 

 I mean the placenta on the midi'ib of the carpel, as in 

 Ophrys apifera, Fig. 110. Possibly it may have been 

 taken too much for granted that parietal placentae are 

 always marginal, but it is evident to me that such 

 placentae may often be on the midrib, M'hile the margins 

 of the carpels may have fused and left no trace of 

 suture, as I think has been the case in the peel carpels, 

 or divisions of the citrus. 



