236 THE ANATOMY OF WOODY PLANTS 



The phenomenon of recapitulation is not confined, however, to 

 external features of organization, for it is often equally well exempli- 

 fied by internal anatomical structure. A good illustration of the 

 principle of recapitulation is presented by the seedling of the 

 araucarian conifers. The adult stem of the kauri, or of any other 

 araucarian conifer, is characterized by two unique features. One 

 of these is the persistence of the traces belonging to the leaves long 

 after the foliar organs have fallen. The foliar fibro vascular strands 

 are continued for many years, amounting in some cases to centuries, 

 through the activity of the cambium, even when the surface of the 

 trunk has long ceased to show even the scars of the leaves of which 

 they were once the fibrovascular supply. Further, in araucarian 

 woods there is present a peculiar variety of tracheary structure 

 which clearly differentiates their ligneous organization from that of 

 all other living subtribes of conifers. The tracheids in the Arau- 

 cariineae have their bordered pits arranged in an alternating manner 

 and not disposed in an opposite fashion, as is the characteristic 

 condition in the rest of the living conifers. In araucarian woods of 

 the Mesozoic belonging to the genus Brachyoxylon the leaf traces 

 persist only for a short time and are no longer formed through the 

 instrumentality of the cambium after the leaves to which they 

 belong have fallen from the stem. Again, the pits do not manifest 

 the alternating and crowded condition presented by the wood of the 

 living genera. In the seedling of both Agathis and Araucaria 

 the leaf trace persists only so long as it is related to a functional 

 leaf, and does not continue to develop for many years after the fall 

 of the foliar organs, as is the case in the older trunk. Also in the 

 araucarian seedling the pitting is like that found in the Cretaceous 

 araucarian genus Brachyoxylon. In this instance we have a striking 

 exemplification of the principle of recapitulation. 



The law or principle under consideration has many illustrations 

 in the vascular plants, but on the whole it cannot be said to have 

 so great a validity as among the higher animals. It is further 

 necessary to note in the present connection that the absence of a 

 given structure in young individuals is by no means evidence 

 of its absence in the ancestral forms from which they have been 

 derived. For example, there is good evidence that the cycadean 



