THE EQUISETALES (INCLUDING SPHENOPHYLLALES) 267 



clearly shown. The wood is apparently entirely secondary in its 

 origin, but it is distinguished by certain lacunae or cavities which 

 occupy the apices of the wedgelike masses constituting the rather 

 slender cylinder under discussion. These cavities indicate the posi- 

 tion of the poorly differentiated and evanescent primary wood. So 

 far as is known, the primary xylem of the calamites of the later 

 Paleozoic was entirely centrifugal or peripheral in its development. 

 The great reduction in the primary structures of the calamites has 

 led to a discontinuity of the cylinder comparable to that found in 

 the higher and more modern members of the Sigillariae described in 

 the preceding chapter. The lack of continuous development in the 

 cylinder of primary wood brings with it a corresponding organiza- 

 tion of the secondary xylem. The general result of the situation 

 portrayed is the organization of the secondary cylinder in the first 

 instance as separate wedges which finally become united by their 

 increasing breadth. In the group under consideration we have 

 one of numerous examples of an interrupted secondary cylinder 

 resulting from the meager and sporadic development of the primary 

 wood when the latter presents an extreme degree of reduction. 

 The pointed, outward excursions of the pith in calamitean forms 

 are in marked contrast to the true medullary rays, which in this 

 case are narrow structures. The rays in Calamites are characterized 

 by the often vertical elongation of their elements, a situation which 

 parallels that described above in Sphenophyllum. 



The slight development and the entirely centrifugal origin of 

 the primary xylem in the true calamitean forms is in marked 

 contrast to the conditions presented by the genus Sphenophyllum, 

 where the primary structure is not only massive, but also entirely 

 centrad or centripetal in its development. There is, of course, 

 a very wide gap between the organization of the axis in Spheno- 

 phyllum and that found in Calamites. This gap is for the most 

 part still unbridged by the discovery of intermediate forms, but 

 an interesting condition is found in a stem from the lower Car- 

 boniferous which is described by Scott. In Fig. 190 is repro- 

 duced a somewhat oblique section of the primary region of one of 

 the woody wedges of a calamitean stem. The lacuna or cavity 

 representing the position of the ephemeral primary wood has 



