STRUCTURE OF EOUISETUM 



311 



erect shoots are the upturned ends of the branches of horizontal 

 underground rhizomes (Fig. 174, C), \\'hich bear similar whorls 

 of scale-leaves, as well as numerous adventitious roots, at each 

 node, the primary root dj'ing away at an early stage, as in Ferns. 

 The rhizome of the Field Horsetail (Eqnisetum arvense) bears, 

 in addition, tuber-like storage-organs formed from modified 

 branches. In the ancient Horsetails [Calamitcs) of the Palfeozoic, 

 leaves (cf. Fig, 175, B) and branches were likewise whorled. 



Fig. 174. — A, Section of stem of Equisetum palnslre (after Pfitzer), showing 

 tlie numerous air-spaces (shaded) and the ring of vascular strands (v.). 



B, Upright, fertile shoot of E. pahistre, bearing a terminal cone (Sp.). 



C, Habit of E. limosum, showing the subterranean rhizome, with 

 numerous adventitious roots arising from the nodes and t^vo aerial 

 shoots [a.]. SI., whorls of scale-leaves. (B and C original.) 



The anatomy of the stem superficially resembles that of a 

 Dicotyledon in the arrangement of the vascular tissues, and 

 invariabty shows great reduction of the xylem and a conspicuous 

 system of air-canals in the cortex and pith (Fig. 174, A). These 

 aquatic characteristics are to be expected in the case of partially 

 submerged species like Eqitisetiim liniositm and E. paliisfre, but 

 the fact that others (e.g. E. arvense), growing in relatively dry 

 situations, nevertheless exhibit these features indicates that the 



