250 THE ANATOMY OF WOODY PLANTS 



with the large-leaved Pteropsida must, for the present at any rate, 

 be left an open question. Although the Lycopsida were the pre- 

 dominant constituent of the Paleozoic forests, the Pteropsida in 

 many cases have entered largely into the composition of the more 

 ancient coals and can often be clearly recognized, particularly in the 

 carboniferous coals of the Middle Western states (Illinois, Ohio, 

 etc.), as charred remains of the axes and even as pinnae or smaller 

 segments of the leaves in the form of so-called "Mother of Coal." 



After having described the general characteristics of the Lycop- 

 sida and Pteropsida, we find it in order to indicate the main groups 

 which come under these two great divisions of the vascular plants. 

 The Lycopsida include two principal subdivisions the Lycopo- 

 diales and Equisetales. The Lycopodiales are characterized by 

 the alternating nature of their foliage, while the Equisetales have 

 their leaves disposed in whorls on a stem presenting marked ridges 

 and furrows. The Lycopodiales are again subdivided into isos- 

 porous and heterosporous families. Of the former there are two 

 the Lycopodiaceae and the Psilotaceae. The Lycopodiaceae are 

 characterized by the possession of well-developed roots and un- 

 divided sporangia, while in the Psilotaceae the sporangial struc- 

 tures are septate, and organs of the nature of roots are entirely 

 absent. The heterosporous Lycopodiales have in common a foliar 

 appendage, known as a ligule, which is present on both vegetative 

 and reproductive leaves, and in the case of the latter occurs just 

 above the insertion of the sporangium. Of the three families 

 presenting the phenomenon of heterospory the first, the Selaginel- 

 laceae, are terrestrial forms included under a single genus, Selaginella, 

 in which the megasporangia never produce more than four spores. 

 In the second and usually aquatic family, the Isoetaceae, there is 

 likewise a single genus, but the sporangia are provided with trans- 

 verse processes known as trabeculae, and the megaspores are 

 numerous in each sack. Lastly, the Lepidodendraceae, including 

 the Sigillariae and their allies, are terrestrial extinct forms often of 

 arboreal habit and of somewhat diverse megasporangial structures. 



The Equisetales, as has been indicated at the beginning of the 

 foregoing paragraph, are distinguished by the whorled arrangement 

 of their leaves. Another feature which they possess in common is 



