THE MAIN LINES OF PLANT EVOLUTION 



Figure 40. A Horsetail, Equisetum. (From Weather- 

 wax, "Plant Biology," 2nd Ed., W. B. Saunders Co., 1947. ) 



the latter, they probably arose in the Devonian from psilophyte ancestors 

 by the differentiation of the body into definite root, stem, and leaves. The 

 stem of the horsetails has a pith region which is commonly hollow. The 

 stem grows by means of concentrations of meristem at definite nodes, 

 which can be easily disjointed, to the delight of many a child (Figure 

 40). The leaves are arranged as whorls, and they are relatively small. 

 Nonetheless, the weight of opinion favors their origin by flattening of 



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