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BOTANY OF THE LIVING PLANT 



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 Dryopteris, the inner are accessory or medullary meristeles (Fig. 

 376). Each one is circumscribed by a complete endodermis. This 



Fig. 377. 

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

 e= endodermis. ph = phloem with sieve-tubes. *y = xylem, with large scalariform 

 tracheides. Some smaller tracheides lying centrally are the proto-xylem. Note 

 that no intercellular spaces are seen within the endodermis. ( x 75.) 



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

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

 itself. A transverse section of one of them, examined under a high 

 power, gives the following succession of tissues (Fig. 377). Passing in- 

 wards from the starchy ground-tissue, with intercellular spaces (g), the 

 layer of brownish cells of the endodermis (e) forms a continuous barrier, 

 delimiting the strand sharply. Within it follows the pericyde, with 

 its cells not very regularly disposed, but corresponding roughly to the 

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

 single layer. Within this comes the phloem (ph), with large sieve-tubes 



