318 



CALAMITES. 



[CH. 



regularl}^ arranged (fig. 78, 2) and rather thick-walled cells ; 

 this consists of periderm, a secondary tissue, which has been 



Fig. 78. 1. Transverse section of a thick Calamite stem. 



p, pith; X, secondary wood; c, bark, (f nat. size.) 



2. Periderm cells of bark. 



3. Xylem and medullary rays. (2 and 3, x 80.) 



From a specimen in the Williamson Collection (no. 79). 



developed by a cork-cambium during the increase in girth of 

 the plant. The more delicate cortical tissues have not been 

 preserved, and the more resistant portion of the bark has been 

 broken up into small pieces of corky tissue, among which are 

 seen numerous Stigmarian appendages, pieces of sporangia and 

 other plant fragments. These associated structures cannot of 

 course be shown in the small-scale drawing of the figure. 



In the radial longitudinal section (fig. 79) we see the pith 

 w^ith the projecting wood and the remains of a diaphragm at the 



