CHAPTEE XIII. 



THE STELE OF THE PTERIDOPHYTA. 



PRINCIPAL MATERIALS USED. 



Rhizomes or petioles of Pteris aquilina ; in alcohol. 



Stems of Lycopodium sp., e.g., L. complanatum ; in alcohol. 



Barren stems of Equisetum arvense, or other sp. ; fresh, or in alcohol. 



PRINCIPAL REAGENTS USED. 

 Aniline blue Safranin. 



The Rhizome of Pteris Stelar Theory. In our studies of the 

 structure of stem and root of Phanerogams, we have used the 

 word stele as being to all intents and purposes equivalent to the 

 term central cylinder, in which sense Van Tieghem, who intro- 

 duced the term, used it. The stele then consists, in the cases 

 we have studied, of more or less numerous vascular bundles, 

 variously arranged with regard to one another, but alike in that 

 the whole system of bundles is surrounded by a pericycle (in 

 which subsequent developments of the most diverse kinds may 

 originate) ; and that the pericycle, the most external tissue of the 

 stele, is very often cut off from the general body of the surrounding 

 cortex by a specialised layer of this latter, the bundle sheath, or 

 endodermis ; though we have also seen that the endodermis 

 varies very greatly in respect to its recognisability sometimes 

 very distinguishable (e.g., most roots), sometimes forming a 

 " starch layer," sometimes not recognisable as a separate layer 

 at all. In most of the Vascular Cryptogams the stele presents 

 itself to us under an aspect which differs greatly from that of 

 Phanerogams, at first sight often simulating a vascular bundle ; 

 and we will take as our first object of study of the stele in their 

 stems an exceedingly common fern, the Bracken (Pteris aquilina}. 



(172) 



