I2I0 A TEXTBOOK OF THEORETICAL BOTANY 



These arguments for the association of the two types of structure apply with 

 even greater force in the Pteridophyta, where the differentiation between 

 fertile sporophylls and sterile foliage leaves is much less marked than in the 

 Angiosperms. If therefore, as is generally accepted, the foliage leaves of 

 Pteropsida are derived from flattened branch systems, so must also be the 

 sporophylls. To equate a carpel to a sporophyll is not therefore inconsistent 

 with the supposition that it had ultimately its origin in a branch system. 



We have already seen that similar arguments have been advanced with 

 regard to the stamen, and the same difficulties which we then noticed apply 

 to the present case. Granting that originally all appendages wxre telomes on 

 an undifferentiated shoot, the different views held really turn upon whether 

 the organ passed through a foliage phase on the way to its present form, or 

 did not. It is possible that the stamen was never laminar, but the evidence 

 in the case of the carpel is stronger. The carpel receives typically three, 

 sometimes five traces, but with very few exceptions never a single trace 

 like the stamen. This accords with what Eames has shown to be the primi- 

 tively three-trace foliage leaf. The suggestion is that the immediate 

 ancestor of the carpel was a palmate, three-lobed, dorsiventral, leaf-like 

 structure, which was in turn derived from a flat, dichotomous shoot. 

 Hunt has distinguished two forms of carpel, based on this theory. In one, 

 the central lobe, with only one bundle, forms the style and stigma and the 

 outer wall of the carpel, the other two forming the lateral walls. In the 

 other all three lobes contribute to the formation of the style and stigma, 

 the latter being consequently three-lobed, while the style contains three 

 vascular bundles. 



A further theory of the carpel should here be mentioned, put forward 

 by Hamshaw Thomas. He, like Eames, interprets the carpel as a triple 

 structure (Fig. 1179). The dorsum he considers as formed from the axis 



Fig. 1 179. — A, Yasculation of Delphinium carpel. B, Diagram illustra- 

 ting Thomas' theory of the derivation of a simple follicle from a 

 palmate sporophyll. The dorsal, axial segment is shaded. The 

 two lateral segments aborted bv contact are shown with dotted 

 lines. {After H. H. Thomas.) 



