FLORA OF THE PALEOZOIC PERIOD. 31 



and 21). At the present day some of these plants in tropical 

 regions have stems of 15 or 16 feet high. 



Among vascular Acrogens is included the 

 natural order Marsileaceas or Rhizocarpcc^, the 

 Pepperworts (Fig. 22). The order consists of 

 aquatic plants, with creeping stems, bearing 

 leaves, which are either linear, or divided into 

 three or more wedge-shaped portions not unlike 

 clover. The fructification is at the base of the 

 leaf-stalks, and consists of sacs (sporocarps) con- 

 taining spores of two kinds, microspores and 

 macrospores. The order contains Marsilea, Pilu- 

 laria, Azolla, and Salvinia. 



For a fuller account of Acrogenous plants, 

 see Balfour's Class Book of Botany, p. 954. ^^^ 



These orders are represented in the Pala3ozoic 

 flora. Many of the fossil species assume a large 

 size, and show a greater degree of development 

 than is seen in their recent congeners. The most 

 important coal plants belong to the Ferns, Lyco- 

 pods, and Horse-tails. The examination of the Fig. is. 

 structure and conformation of the plants of the present flora 

 assists much in the determination of the fossil carboniferous 

 flora. 



In the lower Pala3ozoic strata the plants which have been 



detected are few^ In the Silurian and Cambrian systems, we 



meet with the remains of ancient marine plants, as well as a 



few terrestrial species. Even in the still older Laurentian 



rocks, if the remarkable structure known as Eozoon canadense 



Fig. 18. Fructification of Equisetum maximum, Great Water 

 Horse-tail, showing the stalk surrounded by membranous sheaths, s s, 

 which are fringed by numerous processes called teeth. The fructifica- 

 tion, /, at the extremity, is in the form of a cone bearing polygonal 

 scales, under which are spore-cases containing spores with fila- 

 ments. 



