CH. IX] 



THE LEAF-TRACE 



i6i 



comparative value, but the most important region for phyletic comparison 

 is the first, that is, the conservative basal reeion. 



The Leaf-trace 



The leaf -trace in types recognised on other grounds as relatively primitive 

 is undivided, consisting of a single vascular tract enclosed habitually by 

 endodermis. The form of it as seen in transverse section varies from the 

 circular to some flattened or variously curved outline. An almost cylindrical 

 leaf- trace with one enclosed tract of protoxylem is seen in Botryopteris cylin- 

 drica (Fig. 153, ^). In this case it closely resembles the vascular supply to a 



Fig- 153- ^ = diagram of stem-stele Fig. 154. Transverse section of shoot of 7>/<5/- 



and petiole-trace of Botryopteris caulis soletiites, Cotta, after Stenzel, show- 



cylindrica, showing undivided ing stem, with protostele, and the last four 



protoxylem in he joval leaf-trace, leaf-traces traversing the cortex. The leaves 



which is the smaller and upper are numbered in their succession. The draw- 



tract in Fig. A, the lower and ing is simplified by omission of roots, etc. • 



larger being the axis. (After Miss 

 Bancroft.) iS = petiolar meristele 

 of Clepsydropsis ; C= foliar trace of 

 Aitachoropteris. (After P. Bertrand.) 



branch of the axis itself, and a siinilar condition is seen in the leaf-trace of 

 many living Hymenophyllaceae. Oval or slightly flattened leaf-traces are 

 found in many Botryopterideae and Zygopterideae (Fig. 153,^), and in the 

 Ophioglossaceae: and the same type is seen at the extreme base in the 

 Osmundaceae (Fig. 155, 2). The outline of the trace in Ferns that are rather 

 more advanced than these varies according to the form of the upper leaf, and 

 as this is usually flattened, the leaf-trace takes the shapeof astill more flattened 

 strap. As the trace passes outwards through the cortex it may become 

 curved, as seen in section, and it is noteworthy that the concavity may be 

 abaxial or adaxial. The former appears in certain Botryopterids and Zygo- 

 pterids, a fact which suggested to Professor P. Bertrand the descriptive name 



B. II 



