i^ FILICINE^ LEPTOSPORANGIATM 327 



Marattia. As the stem grows, the central stele, which at first 

 is solid ("protostelic"), becomes a hollow cylinder ("siphonos- 

 tele"), which, according to Jeffrey (3) in most Polypodiaceae 

 shows a concentric structure, i. e., there is a central mass of 

 wood, with both outer and inner phloem, and an external and 

 internal endodermis. Sometimes, however, e. g., Davallia 

 stricta, both internal endodermis and phloem are absent, and 

 this would seem to be the case 

 also in Struthiopteris (Camp- 

 bell (i)). 



A cross-section of a plant 

 of the latter species with three 

 fully-developed leaves showed 

 the vascular cylinder to be oval 

 in outline, and consisting of the 

 following parts. A central pith 

 of elongated parenchymatous 

 cells, surrounded by a thick ring 

 of short spiral and reticulate 

 tracheids, outside of which was 

 a zone of phloem, the whole 

 enclosed by a distinct endoder- 

 mis. The latter is continuous, 

 with the endodermis of the bun- 

 dles going to the leaves and 

 roots, and the xylem of these 

 also connects with that of the 

 stem bundle. The apex of the 

 stem becomes more and more 

 hidden by the development 

 of scales from the epidermis, 

 which finally completely hide it 

 and form a very efficient pro- 

 tection. 



The petioles of the first three leaves have a single axial 

 vascular bundle, but in the fourth, as in all subsequent ones, 

 there are two. They separate very soon after leaving the stem 

 bundle, which is deeply cleft where they issue from it. These 

 bundles are typically concentric in structure, and have a well- 

 developed endodermis. The number of roots in the young 



Fig. 180 



■Adiantum pedatuTti, A, Rhizome 

 with young leaf, /, and the base of an 

 older one; x, stem-apex. B, leaf-seg- 

 ment, showing venation, and sori, s. 



