THE GENUS EQITISETUM. 163 



merely as broad, roiiiidi'd, longitudinal elevations on the cast between the depressions 

 above mentioned. At one side of the nodal circular depressions and on the ends of the 

 rounded ridges corresponding to the primary medullary rays, there occur frequently 

 knob-like projections. Sometimes less marked tubercles appear in a similar position ou 

 the other side of the node. 



Brongniart (Hist, des vegetaux fossiles) who did not realize that he was dealing 

 with casts, explained the larger nodules as representing rudimentary branches and roots 

 (p. 109), and curiously enough considered them to occur on the upper side of the node, 

 although he was clearly aware that in Efpiisetmn jiuviatile, which he used as the biisis of , 

 his comparison, the roots and blanches come off below the articulations, and alternately 

 with the leaves (p. 103). The other and often absent zone of tubercles he explained as 

 undevelo])ed leaves (Brongniart, o/). clt.) . His ideas concerning the rudimentary' char- 

 acter of the organs represented by the tubercles probably originated from the observation 

 that in specimens, which are otherwise apparently well preserved, the nodules in question 

 are frequently entirely absent. Subsequent investigations, which need not be detailed 

 here, established the fact that in branching rhizomes, the orientation of which can be 

 inferred, the more conspicuous nodules of the cast occur below the node and at the upper 

 end of the loucjitudinal rida-es. 



It has not been so easy to settle the nature of the anatomical features of the calami- 

 tean woody cylinder, which gave rise to the ring of more prominent tubercles. In photo- 

 graph 4 (PI. 27, fig. 4), which is copied from Williamson (Phil, trans, roy. soc, 1.S71, pi. 

 26, fig. 22), we see a tangential section of the primary bundles of a Calamite, which is so 

 close to the medulla that the carinal canals of the protoxylem of the bundles are laid 

 open. It is to be noticed, that the course'of the bundles is the same as in E([uisetum, 

 except, that on the left of the figure no alternation takes place at the node. Above the 

 node are represented the vascular strands belonging to branches, or as has been stated 

 more recently (Williamson and Scott, Phil, trans, roy. soc, 1804, B., p. 876) to leaves. 



It is to be observed that these writers assume for the Calamites a different relation 

 of the branches to the nodes, from that obtaining in Equisetum, i. e., that they originated 

 above the node and not from the node, as in the latter (Williamson and Scott, op. cit., p. 

 890). In the medullary rays, below the nodal wood, the parenchyma is seen in spots I 

 to be somewhat disintegrated. In photograph 5 (PL 27, fig. 5) which is a copy of a 

 figure (Williamson, Phil, trans, roy. soc, 1878, plate 20, fig. 23) representing a more 

 external longitudinal section passing through the secondary wood, may be seen the 

 branch-traces m (or leaf-traces as they were later called) no longer running free in 

 the upper medullary rays, but arched over more or less by strands of secondary wood. 

 In the lower rays disintegration has gone so far that actual cavities, I, have made their 



