X] CONES OF CALAMITES. 349 



and Scott 1) immediately opposite each protoxylem strand. In 

 the roots of recent plants {e.g. Gucurbita, Phaseolus, &c.) a 

 broad medullary ray is often found opposite the protoxylem, 

 and such an arrangement is a perfectly normal structure in 

 roots ''. 



Renault has recently described several species of Calamite 

 roots which he designates by specific names, some of them 

 belonging to stems with the Arthropitys structure, and others 

 to Galamodendron. Some of the roots figured by the French 

 author have an axial strand of xylem with 7 — 15 projecting 

 angles of protoxylem'. These he considers true roots, but the 

 larger specimens with a wide pith he prefers to regard as stolons. 

 In the latter he mentions the union of the primary centri- 

 petal with the secondary centrifugal wood as a distinguishing 

 feature. It has been shown, however, that each group of 

 secondary xylem includes a median ray of parenchyma, and that 

 the whole structure is essentially that of a root, and not that of 

 a modified stem or stolon. The organs described by Renault 

 as true roots are probably rootlets, and as Williamson and 

 Scott have demonstrated, there is every gradation between the 

 smaller specimens with a solid xylem axis and those with a 

 large central pith. 



It is interesting to note that Renault's figures of Galamo- 

 dendron roots show the closest resemblance to those of the 

 subgenus Arthropitys. 



d. Gones. 



The occurrence of fossil plants in the form of isolated 

 fragments is a constant source of difficulty, and is well illustrated 

 by the numerous examples of strobili which cannot be con- 

 nected with their parent stems. We are, however, usually 

 able to recognise Calamitean cones if the impressions or 

 petrified specimens are fairly well preserved, but it is seldom 

 possible to coiTolate particular types of cones with the corre- 

 sponding species of foliage-shoots or stems. Palaeobotanical 



1 Williamson and Scott, loc. cit. p. 689. 



= de Bary (84), p. 474; van Tieghem (91), p. 720. 



' Renault (96), pp. 118, 126; (93), PI. lv. 



