170 



HISTORICAL GEOLOGY 



record doubtless being the very favorable conditions for pres- 

 ervation of the flora of the time. Several thousand species of now 

 extinct plants are known from the Coal Measures alone. It must 

 be remembered that most of the important classes of Pennsylva- 

 nian plants existed as early as in the Devonian, but these earlier 

 records are much more scant. The known Coal Measures flora 

 consists almost entirely of the higher Cryptogams (Pteridophytes) 

 and the lower Phanerogams (Gymnosperms) , though Thallophytes 

 (e.g. Algae) certainly, and Bryophytes probably, also existed. 



From the negative standpoint, 

 the most significant feature was 

 the complete absence of the 

 typical flowering plants (Angio- 

 sperms) which are today the 

 most common and the most 

 advanced of all plants. 



Lycopods (giant Club-mos- 

 ses) were the largest, most 

 abundant, and conspicuous of 

 the forest trees, and they ap- 

 pear to have culminated during 

 this same period. In marked 

 contrast to such a high posi- 

 tion, their descendants of today 

 are represented only by a few, small, delicate, trailing so-called 

 Club-mosses and Ground-pines in our forests. Two of the most 

 prominent of the Pennsylvanian Lycopods were the Lepidodendrons 

 and the Sigillarians. The Lepidodendrons (" scale-trees") had leaf- 

 scars or scales arranged spirally around the trunks of the trees (Fig. 

 98a). They generally attained a height of 50 to 100 feet and a di- 

 ameter of 2 to 4 feet. The tall trunks were slender and they 

 branched dichotomously (by twos) only at a considerable height. 

 Long, stiff, needle-shaped leaves were thickly set on the branches. 

 The dropping of the leaves from the older (trunk) portions caused 

 the leaf -scars or scales above mentioned. Inside of the outer bark, 

 the stem consisted of pithy or loose cellular tissue. Over 100 

 species of the Lepidodendron are known. The Sigillarians (" seal- 

 trees") are so called because of seal-like markings (Fig. 98b) 

 which were arranged vertically on the tree trunk. They were even 

 larger than the Lepidodendrons, having attained a height of 100 



Fig. 98 

 Lepidodendron bark (a) and Sigillarian 

 bark (6), showing arrangement of 

 leaf scars. 



