280 



ORGANOGRAPHY. 



from tlie axis of the cone, which he calls a placenta, and the 

 scales, or bracts, which are situated between them, he believes 

 to be open carpellary leaves. Schleiden also states, that no 

 satisfactory explanation can be given by the advocates of the 

 mai-ginal theory of placentation, of the formation of the ovule 

 and placenta in Armcria, in which the ovary composed of five 

 carpels surrounds a single ovule, which rises from the bottom 

 of the axis, supported on a stalk which 

 curves downwards at its apex, and 

 thus suspends the ovule free in the 

 centre of the cavity {fig. 623). He 

 accordingly concludes, that the ovule 

 and placenta are developments of the 

 axis. Many other arguments in fa- 

 vour of the universal applicability of 

 the axial theory in the formation of the 

 placenta have been brought forward 

 by Schleiden, and others, but their fur- 

 ther discussion would be out of place 

 here. 



From all that has been stated, we 

 may perhaps be allowed to draw the 

 following conclusions, namely : — that 

 no one theory sufficiently accounts for 

 the production of the placenta in all 

 cases ; but that the axile and some 

 forms of the free central placentation 

 may be explained on both hypotheses ; 

 that the parietal placentation is best explained upon the mar- 

 ginal theory ; and that the formation of the free central placenta 

 of the Primulacese, Santalacese, and some other plants, can only 

 be satisfactorily explained by considering the placenta as a pro- 

 duction of the axis. 



In a practical point of view, the mode of production of the 

 placenta is of little importance. The accurate discrimination of 

 the different kinds is however of much value in descriptive botany, 

 by affording us constant, and hence important characters, for dis- 

 tingiiishing plants. Some natural orders exhibit more than one 

 kind of placentation, and hence cannot be distinguished by any 

 particular kind ; in such orders, therefore, the placentation can 

 only be applied in obtaining good characteristics of certain 

 genera. In the majority of instances, however, we find one kind 

 of placentation occurring throughout all the plants of a particu- 

 lar natural order. Thus, the Scrophulariaceae, Ericaceae, and 

 Campanulacese, present us with axile placentation ; the Papave- 

 racepe, Violacese, and Cruciferse, with parietal ; and the Caryo- 

 phyllaceae, Santalaceae, and Primulacese, with free central, 



2. The Style. — We have already described the general nature 

 and structure of the style in speaking of the carpel. There 



Fig. 623. Vertical section of 

 the flower of Armcria. The 

 ovary is seen to contain Init 

 a single oviUe suspended 

 from a funiculus or stallc. 

 The ovule is said to be recli- 

 nate. 



