240 THE ANATOMY OF WOODY PLANTS 



been apparent. Of more recent origin is the realization that, in 

 modern plants at any rate, the root is the most valuable of all 

 organs from the standpoint of evolutionary anatomy. In the 

 case of comparisons between Paleozoic and Mesozoic groups the 

 root, in fact, has proved to be in general too conservative to furnish 

 significant examples of the retention of ancestral characters. The 

 search for centripetal or cryptogamic wood has been in the fore- 

 ground in investigations bearing on the relationship between 

 Paleozoic and Mesozoic groups. Since in all forms of every geo- 

 logical age the root has centripetal primary wood, in regard to this 

 crucial feature of more ancient types it obviously does not supply 

 distinctive evidence in connection with the doctrine of descent. 

 In other respects, however, and particularly with reference to the 

 organization of the tissues of the secondary wood, the root has 

 proved itself to be of greater significance than any other organ of 

 the higher plants. Of course the most striking evidence of con- 

 servatism in the root has resulted from the comparison of Mesozoic 

 and modern forms. It has, for example, been shown that the root 

 in modern conifers clearly and persistently perpetuates the features 

 of organization which characterized the structure of the stem in 

 the Cretaceous and earlier periods of the Mesozoic age. In Pinus 

 and Agathis the root presents in many of its earlier annual rings 

 the distinctive organization found in the stem of these types 

 in the Mesozoic. Nor is the inherent conservatism of the root of 

 value in the study of the gymnosperms alone. Although our 

 knowledge of the anatomical structures of the angiosperms in 

 Mesozoic times is as yet extremely inadequate, we are able in 

 many instances by the application of the doctrine of the con- 

 servatism of the anatomical organization of the root to infer the 

 ancestral type of stem in the highest vascular plants. 



It will be evident from the foregoing paragraphs that conserva- 

 tism is particularly inherent in the leaf and root of vascular plants 

 and that the highly progressive stem presents only features which 

 are of interest from the standpoint of the doctrine of descent, either 

 in its first annual ring or in axes specially allocated to the function 

 of reproduction. The sporangium is a fourth structure recognized 

 as a primitive organ of vascular plants in an earlier chapter of the 



