282 



THE ANATOMY OF WOODY PLANTS 



condition in which no medullary tissues are present, a phase 

 followed by the siphonostelic one, which in turn develops medullary 

 strands from the inner surface of its walls. The outer series of 

 bundles is consequently antecedent to the medullary bundles, a 

 situation which may be readily inferred without the study of 

 the young plant from the fact that the roots are attached to the 

 external strands. Not only does the bracken manifest the general 



FIG. 202. Diagram to illustrate the development of the stem in Pteris aquilina. 

 Explanation in the text. 



conditions described above, but all ferns with complicated arrange- 

 ments of conducting strands in the stem can readily be included 

 under the same general statement. This, for example, is true 

 of the complex stem of the Cyathaceae or tree-ferns and also of 

 that of the large tropical ferns known as the Marattiaceae. It is 

 apparent from the account supplied in the present connection 

 that the tubular condition is both typical and primitive for the 

 ferns in general, with the sole exception of those forms in which 

 the organization of the stele maintains the original protostelic 

 condition. This type of structure is, however, comparatively 

 rare in existing ferns, although it has been found, as would 



