ORGANS OF REPRODUCTION. 



287 



Schleiden, indeed, and some other botanists regard the pla- 

 centa in all cases as a development from the axis of the plant. 

 Schleiden, particularly, has written an elaborate paper in support 

 of this view. The axile and free central placentution are 

 readily to be explained by it, but the formation of the parietal 

 placenta is by no means so clear. It is supposed that the axis 

 ramifies in the cavity of the ovary, and that the branches curve 

 directly from their origin towards the side, and become blended 

 with the margins of the two carpels on their inner side, and 

 form parietal placentas bearing the ovules as lateral buds. 

 Schleiden thinks, that the formal ion of the ovule in the Yew, 

 where it terminates a branch, and is naked, is incompatible with, 

 the marginal theory. He thinks, also, that the formation of the 

 ovules generally in the Coniferse, support his views of placent- 

 ation. He regards the ovules in those plants as being given otf 

 from the 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 

 marginal theory of placentation, of the formation of the ovule 

 and placenta in Armeruu 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. 622). 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. For additional particulars, I must 

 refer the reader to Schleiden's Prin.' 

 ciples of Botany, translated by Lankes- 

 ter, and to Lindley^s Introduction to 

 Botany. 



From all that has been stated, we 

 may perhaps be allowed to draw the 

 following conclusions, namely : — that 

 no one theoiy sufliciently 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 marginal theory ; and that the formation of the free 

 central placenta of the Primulacea^, Santalacete, &c., can only be 



Fig. 622. 



Fig. 622. Vertical section of 

 the flower of JrmtcfVt. The 

 ovary is seen to contain but 

 a single ovule su:ipen(lea 

 from a funienlus. The 

 ovule is said to be reclinate. 



