CHAPTER XX 

 THE EQUISETALES (INCLUDING SPHENOPHYLLALES) 



The forms to be discussed in the present chapter are contrasted 

 with the Lycopodiales by the general fact that the appendages of 

 the stem are arranged in a whorled or verticillate manner instead 

 of in the spiral fashion characteristic of the group considered in 

 the preceding chapter. Not only, however, are they characterized 

 by the verticillate arrangement of the appendages, but also by the 

 longitudinal ridges and furrows which mark the surface of their 

 stems. Contrary to the conditions found in the furrowed or angular 

 stems of certain woody dicotyledons, the traces or fibrovascular 

 strands of the leaves correspond in position to the salient regions 

 of the stem and not to its depressions. The branches, moreover, 

 are not truly axillary as is characteristic of the higher forms, but 

 occur at the node in alternation with the foliar organs. This 

 situation is very striking and characteristic. The older forms in 

 the Equisetales usually possessed dichotomously divided leaves 

 or at least foliar organs in which the veins repeatedly forked. 

 Another important characteristic of the older representatives of 

 the Equisetales was the continuity of the ridges and furrows at 

 the nodes. In more modern types this condition gives place to 

 distinct and finally universal alternation of the ridges and furrows 

 in the nodal region. 



The Equisetales may be conveniently divided into three groups 

 the Sphenophyllaceae, the Calamitaceae, and the Equisetaceae. 

 Of these the first-named may now be discussed. The Sphenophyl- 

 laceae are Paleozoic forms with slender stems marked by the pres- 

 ence of a relatively small amount of secondary growth. Their 

 slender conformation has led to the suggestion that they were 

 either vines or aquatics. There is, however, no convincing evidence 

 of the correctness of either of these views in regard to their habit. 

 Fig. 188 illustrates the organization of the stem in Sphenophyllum 

 insigne. The center of the cylinder is occupied by the primary 



264 



