242 THE ANATOMY OF WOODY PLANTS 



present in their Mesozoic forebears. For example, in the arau- 

 carian conifers, cited earlier in another connection, the Mesozoic 

 organization of the wood is recalled in the stem of the existing 

 species as a consequence of injury. The wood formed subsequent 

 to the infliction of the injury shows a type of structure charac- 

 teristic of the group in an earlier geological epoch. Although 

 only in rare instances can structures which are actually known 

 to have been present in the past be recalled by injuries, in many 

 other cases we find the infliction of wounds followed by the for- 

 mation of features which are normally present in the conserva- 

 tive organs. Consequently the doctrine of reversion finds support, 

 not only in the facts of paleobotanical anatomy, but also in the 

 much more richly exemplified doctrine of conservative parts. It 

 is further clear that we cannot interpret all structural peculiarities 

 which result from injury as reversions to a former condition of 

 organization, but only those which are definitely paralleled by 

 known conditions in fossil forms or are illustrated in the anatomy 

 of the conservative organs the leaf, reproductive axis, and stem, 

 or at least some of these. 



Carefully as the doctrines of recapitulation and of conservative 

 parts must be applied to elucidate the course of evolution, the 

 principle of reversion must be invoked in phylogenetic studies 

 with even greater precautions. A wide knowledge of fossil forms 

 as well as an extensive acquaintance with the facts of comparative 

 anatomy are necessary for a successful application of this doctrine 

 to the data derived from the investigation of injuries. In this 

 connection it is further necessary to distinguish clearly between 

 the phenomena of hypertrophy and those of actual reversion. For 

 example, when the stem of a dicotyledon or of a conifer is injured, a 

 local damming of food substances results as a consequence of the 

 elimination of the conducting tissues of the phloem in the immediate 

 vicinity of the wound. Overdevelopment consequent on over- 

 nutrition accordingly is locally most conspicuous. Very often 

 true reversionary changes resulting from the impulse supplied by 

 the injury present themselves only at some distance from the 

 wound. This situation is found in the conifers in connection with 

 reversionary conditions following injuries in the case of the rays 



