278 



THE ANATOMY OF WOODY PLANTS 



The stem as the most plastic of the organs in vascular plants 

 presents the greatest variety of structure in the Filicales. The 

 leaf, and particularly the root, offer little diversity of organization 

 and may consequently be dismissed with relatively slight con- 

 sideration. Anatomically the stem presents itself in the case 

 of the Filicales under two main conditions: the protostelic, in 

 which there is no medulla present in the fibrovascular system, 



and the siphonostelic, 

 characterized by the 

 existence of a central 

 mass of parenchyma 

 known as the medulla 

 or pith. The first con- 

 dition is represented 

 in Fig. 199, portray- 

 ing the transverse 

 section of the stem of 

 a species of Glei- 

 chenia. The siphon- 

 ostelic modification is 

 delineated in Fig. 200, 

 reproducing the 

 transverse aspect of 

 the stem of the 

 maidenhair fern, 



FIG. 199. Transverse section of the stem 

 Gleichenia species. 



Adiantum pedatum. In the second figure we find the fibrovascular 

 tissues organized in the form of a tube, limited both internally and 

 externally by an endodermal boundary which becomes continuous 

 around the margins of the gaps caused by the exit of the traces of the 

 lateral branches and leaves. Not only is the tubular central cylin- 

 der bounded continuously by an endodermal layer, but it is likewise 

 characterized in the particular case under discussion by an inner 

 and outer lining of phloem. In the walls of the stelar tube so 

 organized there are gaps formed in connection with the exit of 

 the fibrovascular strands leading to both leaves and lateral branches. 

 In addition to these there may be interruptions in the continuity 

 of the fibrovascular hollow cylinder which are not related to 



