280 THE ANATOMY OF WOODY PLANTS 



their original concentric or bicollateral structure and develop 

 a collateral organization. This condition of the finer fibrovascular 

 structure of the foliar organ is clearly correlated with the dorsi- 

 ventral structure of the leaf as a whole, and favors its function- 

 ing in relation to photosynthesis and transpiration. The upper 

 region of the foliar strands is consequently largely under the sway 

 of physiological conditions, while in their lower course, and espe- 

 cially before they have suffered much in magnitude as a result of 

 subdivision, the ancestral conditions may as a rule be more readily 

 observed. Further, the more aberrant the anatomical structures 

 are in any given case the less will be the degree of development 

 of the ancestral conditions in the foliar traces. In argument 

 regarding the interpretation of the fibrovascular structures in 

 stems the organization of the foliar trace after it has left the 

 stele of the leaf is of great importance. In the root of such 

 relatively low types as the Filicales little evidence is supplied 

 which is of value in the interpretation of the primitive organiza- 

 tion of the tubular stele of the stem. This statement holds, 

 not only for the true ferns, but also for those lower and ancient 

 gymnosperms which have the most marked filicinean affinities. 

 It will be apparent, however, when the discussion of the anatomical 

 organization of the axis in higher gymnosperms and angiosperms 

 is reached, that the root assumes an evolutionary significance 

 which is not observed in the lower groups of the Vasculares. With 

 these preliminary remarks it is possible to pass with advantage 

 to the consideration of the organization and evolution of the 

 tubular central cylinder of the axis. 



First will be discussed modifications of the tubular stele which 

 are in the direction of greater complexity. In many ferns the 

 central cylinder or stele in the adult stem is represented by a 

 complex grouping of strands. In the bracken fern (Pteris aquilina), 

 as is shown in Fig. 201, the older stem presents in transverse 

 section two series of bundles two large central ones and a ring 

 of usually much smaller ones forming a circle outside these. The 

 significance of the conditions present in the stem of this most 

 commonly studied fern have been very generally misunderstood. 

 It has been maintained by the distinguished French anatomist 



