46 STUDIES IN FOSSIL BOTANY 



The cortex is bounded externally by a layer of cells 

 with very thick outer walls, comparable to the exo- 

 dermis of many recent roots (Fig. 16). In the older 

 roots a periderm was formed, the external cells of 

 which, as Dr. M. C. Stopes has shown, thickened their 

 outer walls and replaced the primary protective layer. 



We have now obtained a fairly good idea of the 

 whole construction of a Calamarian plant, as regards its 

 vegetative organs. We have a thorough knowledge of 

 the structure of the stem in various types, and though 

 much has still to be learnt about the comparative 

 anatomy of the leaves, we now know almost the whole 

 story of the structure of the roots and branches, and 

 their relation to the stem. 



7. The Medullary Casts. — One thing remains to be 

 said before leaving the vegetative organs, in order to 

 complete the anatomical explanation of the markings 

 on the casts. We have already seen, as shown in the 

 first illustrations to this chapter, that every Calamitean 

 cast shows longitudinal ridges and furrows. It is quite 

 evident that the furrows on the cast correspond to the 

 vascular bundles. We have to imagine the soft tissues 

 decayed, leaving only the woody tissues behind, so that 

 each wedge-shaped strand of wood leaves its mark on 

 the mineral matter filling the pith -cavity. Between 

 the furrows thus produced we find the projecting ridges, 

 corresponding to the medullary rays which had decayed. 

 Ultimately the wood itself became reduced to a car- 

 bonaceous film, leaving the cast within visible. 



' On a well-preserved Calamitean cast, a little pro- 



