FLORA OF THE PALEOZOIC PERIOD. 



27 



small spores or microspores (Fig. 16), and large spores or 

 macrospores (Fig. 17). They consist of cellular and yascular 



Fig. 9. 



tissues, the latter occurring in the fonn of woody, annular, 

 and scalariform vessels, which occupy the axis or central part 

 of the stem. They differ from ferns in the distribution of their 

 vascular bundles. The order is represented also by such plants 

 as Selaginella, Psilotum, Phylloglossum, and Isoetes. In the 

 plant called Isoetes (Quillwort) there is a peculiar short stem 



Fig. 9. Tree-fern, with a slender cylindrical trunk and a crown of 

 drooping fronds. It is a vascular acrogen. 



