4]0 PRTMARV ARRANGE^[FNT OF TISSUES. 



sclerotic lavers are absent. In the stems of Selasfinella the sclerosis is sometimes 

 limited to the epidermis (S. spinulosa) ; in most species it further affects a hypo- 

 dermal zone, which graduall}' passes over internally into thin-walled parenchyma ; 

 in S. riipestris the entire tissue of the stem is in the highest degree sclerotic, with the 

 exception of the zone of lacunar parenchyma, which in this, as in all other species 

 investigated, directly or indirectly surrounds the vascular bundles. (Comp. Fig. 131, 

 p. 2S2.) The roots of the Selaginell3e and Lycopodia present essentially similar 

 phenomena to those of the stem, with reference to the conditions in question. 



