YOUNG STEM. 



305 



plants must constantly be on his guard against the possible 

 misinterpretation of Stigmarian ' rootlets,' which are frequently 

 found in intimate association with fossil tissues. The intrusion 

 of these rootlets is admirably illustrated by a section of a Cala- 

 mite stem in the Williamson Collection (No. 1558) in which the 

 hollow pith, 2 cm. broad, contains more than a dozen Stigmarian 

 appendages. 



Fig. 71. Transverse section of a young Calamite stem, c, carinal canals ; 

 mr, primary medullary rays; a, h, and d, cortex; e, epidermis. From a 

 section in the Manchester Museum, Owens College, x 60. 



In the figured specimen of a Calamite twig (fig. 71) 

 there is a clearly marked differentiation into a cortical region 

 and a large stele or central cylinder. The pith-cells are already 

 partially disorganised, but there still remain a few fairly large 

 parenchymatous cells internal to the ring of vascular bundles. 

 The few irregular projections into the cavity of the large pith 

 consist of small fragments of cells, which may be the result 

 of fungal action. Mycelia of fungi are occasionally met with in 

 the tissues of older Calamite stems. 



The position of the primary xylem groups is shown by the 

 conspicuous and regularly placed canals, c; these have been 



