5J EVOLUTIONARY DEVELOPMENT AND PAST HISTORY 137 



Horsetails. Outstanding examples are Lepidodendron and Sigillaria, 

 shown in Fig. 32, which were characterized by extensive rooting 

 systems of a unique kind, spores of two sizes, and usually tall and 

 straight, woody trunks covered with the scars left by the spirally- 

 arranged leaves. These last were narrow and grass-like but ligulate, 





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ill 



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hi if 



Fig. 32. — Fossil Lycopodineae. A, Lepickniendioii, two figures on the left, and 

 Sifiillaria, five figures on the right, showing also the characteristic rooting system 

 (after Grand'Eury) (the tallest is about 16 metres in height); B, Lepidodendron 

 lycopodioides, showing the small leaves and leaf scars (after Zeiller) ( < about f ). 



and in Sigillaria are said occasionally to have exceeded 50 cm. in 

 length. Some of the earlier groups quite likely go back to Silurian 

 times, and probably evolved from the psilophytinean complex ; later 

 ones became highly specialized trees, sometimes with seed-like 



