356 OVARY AND CARPELS 



species of Fungi. The Angiosperms first appear in the geological 

 succession in the Cretaceous period, but the study of these and 

 of the abundant Tertiary fossils, though affording much that is 

 of interest, has so far shed no light whatsoever on the origin 

 of the group, which is merged in obscurity. The details of the 

 life-history betray a very high degree of specialisation, but afford 

 practically no evidence as to how this has been attained. 



The ovary of the Angiosperm consists of one or more carpels 

 or megasporophylls , whose nature is most obvious where they 

 are distinct from one another, as in the Marsh Marigold and 

 Pea. Here dissection shows the carpel to have a leaf-like struc- 

 ture, the midrib being marked by a well-defined vascular bundle, 

 whilst the somewhat enlarged edges are joined and form two 

 placentae, each bearing a row of ovules. Except, therefore, for 

 the inrolling, such a carpel might be compared to the megasporo- 

 phyll of Cycas (cf. p. 324). 



Since the ovules are enclosed, the pollen cannot reach the 

 micropyle direct as in Gymnosperms, but is received by the 

 tip of the carpel, which is modified to form a special receptive 

 surface, the stigma. Upon this germination of the pollen takes 

 place. The stigma bears numerous papilla-like hairs, whose 

 secretion may nourish the developing pollen. Very commonly 

 the upper part of the carpel is produced into a stalk-like pro- 

 longation, termed the style, which serves to raise the stigma to 

 a position more suitable for the reception of the pollen. 



When, as is usually the case, several carpels are fused together 

 to form a syncarpous ovary, the number of component carpels 

 may be indicated by the number of loculi or by the number of 

 lobes exhibited by the stigma. In most syncarpous ovaries the 

 ovules retain their position on the margins of the carpels (axile 

 or parietal placentation) , though other arrangements obtain in 

 some cases. Thus, in the Primulaceae and Dock-family (Poly- 

 gonaceae) the ovules are borne on a prolongation of the floral 

 axis (free central placenta of the former), whilst in the Flowering 

 Rush (Butomus) and the White Water Lily (Nymphaa) they 

 occur on the entire inner surface of the carpels. In many Angio- 

 sperms the ovary contains numerous ovules, but the number is 

 not uncommonly reduced to one in each loculus (e.g. Umbellif erae) , 

 or the whole ovary produces but a single ovule (e.g. Gramineae, 



