THE PTERIDOPHYTA : FILICALES, THE FERNS 



541 



LEAF IV. 



LEAF 



LEAF II 



LEAF 



ROOT 



as occur in Marattia. From the base of each leaf arises one adventitious 

 root, which bores its way out through the leaf sheaths of the older leaves. 



The vascular structure of a stem such 

 as that of Botrychium, which is small in 

 relation to the leaves it bears, often pre- 

 sents difficulties of interpretation. It is 

 not clear, for example, how much of it is 

 made up of the leaf traces. At the base 

 of the young plant there is, however, a 

 [)rotostele, surrounded by an endodermis, 

 and this develops at higher levels a 

 medulla, which is sometimes mixed with 

 isolated tracheids (Fig. 541). The leaf 

 trace is a simple arc of vascular tissue, 

 which leaves a considerable leaf gap in 

 the stele. Across this gap an endodermal 

 band is organized, independent of the 

 primary endodermis and often stretching 

 into the pith. It interrupts the continuity 

 between pith and cortex through the leaf STEM APEX 

 gap, though for what reason is not known. 



The xylem consists of very thick- 

 walled tracheids with the protoxylem 

 internal, i.e., endarch, and, what is 

 highly peculiar, traces of a cambium 

 and of secondary vascular tissue are 

 found in old stems. In B. virginianum, 



mentioned above, there is quite a considerable amount of secondary thicken- 

 ing (Fig. 542), a unique case among present-day Ferns, though not uncommon 

 in fossil types. We are justified, therefore, in regarding the genus Botrychium 

 as primitive, with certain very ancient features of structure. 



The Botrychium plant has been interpreted by some morphologists as 

 entirely axial, with four successive dichotomies. The first is the separation 

 of the leaf from the stem and the third is the separation of the fertile and 

 sterile leaf segments. The second and fourth are represented only by the 

 divisions of the vascular strand in the petiole and in the leaf segments 

 respectively. If this view be established, then the so-called " leaf " is an 

 axial branch and the affinity of Botrychium with the fossil Psilophytales is 

 brought much closer. 



VASCULAR 

 BUNDLE 



STEM 



Fig. 540. — Botrychium lunaria. Longi- 

 tudinal section of the stem apex 

 showing successive development of 

 leaves. {After Goebel.) 



Anatomy of the Root and Leaf 



The root contains as a rule one simple collateral strand (Fig. 543). The 

 simple arc of vascular tissue which forms the lower part of the leaf trace 

 becomes a U-shaped band in the petiole. Just below the attachment of the 

 fertile segment of the leaf the band splits laterally into two. Each of the 



