FILTCALES 



331 



vascular arrangement in the leaves of Ferns resembles that in 

 Flowering Plants more nearly than does that of their stems. 



For the study of the tissues composing a vascular strand, a rhizome 

 with long internodes, such as the Bracken, gives the best results. In 

 a transverse section taken between the leaf-insertions an outer and 

 inner series of vascular strands is found, separated by an incomplete 

 ring of sclerenchyma. The outer series corresponds to the mesh-work 

 of Nepkrodium, the inner are accessory, or medullarv meristelcs 



Fig. 270. 

 Part of a transverse section of a meristele of Bracken. g = ground parenchyma. 

 « = endodermis. /)7j=phloem with sieve-tubes. ;cy=xyleni, with large scalariform 

 tracheides. Some smaller tracheides lying centrally are the proto-xylera. ( x 75.) 



(Fig. 269). Each one is circumscribed by a complete cndodcrmis. This 

 is usual in Ferns. Each consists of a central core of xylem, surrounded 

 by phloem ; in fact they repeat the main structure of the protostcle 

 itself. A small part of one of them, examined under a high power, 

 gives the following succession of tissues (Fig. 2;u). Passing inwards 

 from the starchy ground-tissue, with intercellular spaces (.i,^), the layer 

 of brownish cells of the endodennis (c) forms a continuous barrier, dc- 

 hmiting the strand sharply. Within it follows the pericycle, witli its 

 cells not very regularly disposed, but corresponding roughly to the 

 cells of the endodermis, both having been derived by division from a 



