THE FILICALES 



281 



Van Tieghem that the vascular system of the bracken arises by 

 the continued forking of an originally simple (protostelic) strand. 

 According to his view the two large central bundles are the first 

 to appear as the result of the process of forking, and the smaller 

 circle of strands lying outside these is formed later. On account 

 of the supposed origin of the strands in the stem of Pteris aquilina 

 by repeated division, the name polystelic was given by Van Tieghem 

 to this and similar 

 conditions of ana- 

 tomical organization 

 in the ferns and their 

 allies. As a matter 

 of fact the situation 

 is very different 

 indeed from that 

 indicated by the 

 term polystelic. In 

 Fig. 2020, is shown 

 the transverse sec- 

 tion of a very young 

 stem of the bracken 



when It IS Still in the Fi( , 20I- _c om pii ca ted bundle system of Pteris aquilina 

 upright condition 



and has not given rise to the subterranean horizontal branches 

 which come into existence at a comparatively early stage. The 

 stem at this age is obviously siphonostelic and is marked by a gap 

 correlated to an outgoing leaf trace. In & a later stage of 

 development, characteristic of the young horizontal stem, is 

 shown. Here the tubular central cylinder is giving off internally 

 and toward the center a medullary strand. Later the single 

 medullary strand becomes double and the condition attained in 

 the adult is reached. It is clear from an examination of the 

 actual course of development in the stem of the bracken fern 

 that the medullary strands are not formed first, as is assumed 

 by Van Tieghem, nor does the stelar system of the adult result 

 from repeated bifurcation of an originally single strand. On the 

 contrary, the course of development presents first a protostelic 



