GENERAL MORPHOLOGY 



373 



The leaves themselves were usually simple, as in Ec/uisetum, though of 

 greater dimensions, and accordingly more effective as assimilating organs; 

 but among the earliest forms, such as Asterocalamites (Schimper), from 

 the Culm, the leaves were branched in repeated dichotomies (Fig. 199). 

 In the very early Pseudobornia, from the upper Devonian of Bear Island, 1 

 the foliage was forked in a fan-like fashion, and of considerable dimensions. 

 Another feature, in which certain of the earliest forms differed from the 

 later, was in the fact that the members of successive whorls were super- 

 posed, and did not alternate (e.g.. Asterocalamites). Such forms have been 



FIG. 196. 



Equisetum pratense, Ehrh. Shoots showing recurrent whorls 

 of sporangiophores and of bracts. (After Milde.) 



B 



FIG. 197. 



Pltyllotheca. Zigno. A, Ph. eqnisetiforntis 

 from Rovere di Velo, near Verona. B, inflores- 

 cence from Siberia, placed by Schmalhausen with 

 Phyllotheca. (After Solms.) 



associated by Potonie as" a family of " Protocalamariaceae." The facts 

 would seem to indicate then a primitive construction of the Equisetoid 

 shoot as having relatively large \vhorled and superposed leaves, effective 

 as assimilating foliage : these were also separate from one another, and 

 liable to bifurcation. The condition, as seen in the present Equisetum, 

 might be understood as attained by reduction of the coalescent and simple 

 leaves, which became also alternate instead of superposed, while the 

 assimilatory function was relegated almost entirely to the axis. But there 

 is no certain proof that the actual evolution of Equisetum itself was along 

 such a line as this. 



' Nathorst, Z. Foss. Flora d. Folarliinder. i., Lief. 3, Taf. 7, 8. 



