384 THE ANATOMY OF WOODY PLANTS 



It will accordingly be of interest in the present connection to 

 summarize the evidence in regard to the primitive type of dicotyle- 

 don supplied by anatomical data. 



A tendency of long standing is to consider the amentiferous 

 forms as representing a low condition among the dicotyledonous 

 angiosperms. In discussions in this connection it is well to dis- 

 tinguish the true Amentiferae from types which simulate them. 

 The Salicales, for example, on the grounds both of their anatomical 

 organization and of important details of floral structure, cannot 

 be regarded as allied in any but the remotest way with types like 

 the alder and the oak. Further, on anatomical ground types 

 with more or less well-organized floral structure must be admitted 

 to affinity with the Betulaceae, Fagaceae, etc. This is true of 

 the Casuarinaceae and Ericaceae. On the basis of the co-ordinate 

 occurrence of vessels with scalariform perforation, tracheary 

 fibrous elements, diffuse wood parenchyma, and aggregate rays 

 we must accord to the genus Casuarina a primitive position among 

 the dicotyledons. This designation of affinity on the basis of the 

 organization of the wood is supported by the fact that it alone 

 among the dicotyledons possesses transfusion tissue of the flanking 

 gymnospermous type. A further indication of its primitive 

 position is furnished by the presence within the ovules of excep- 

 tionally large amounts of sporogenous tissue and also of tracheary 

 elements. Finally, we have the phenomenon of chalazogamy, 

 the significance of which is still much disputed. On anatomical 

 grounds there could scarcely be stronger reasons for regarding 

 Casuarina as the most primitive representative of the dicotyledons. 

 The only objection that has been seriously urged against this 

 position is the fusion of parts in the ovuliferous floral structures. 

 This objection, however, must weigh lightly in the balance in 

 view of our knowledge of the extreme conservatism of anatomical 

 structures in the case of living and extinct gymnosperms. It has, 

 for example, been pointed out that the living cycads are practically 

 identical in anatomical organization with the extinct bennettitean 

 forms, although their reproductive structures differ very widely. 

 Fusion of floral parts does not furnish a sufficient argument for 

 a high systematic position, as on that ground the genus Welwitschia 



