GROUPS OF PLANTS. 



65 



furnished with two spiral bands or elaters which assist in its 

 dispersal. Some of the Equisetums contain aconitic acid and are 

 used in medicine. Common scouring rush (Equisetum hyemale) 

 is used for polishing woods, and Equisetum arvense (Fig. 45, B) 

 is used for scouring tin ware. 



Fig. 41. Selaginella helvetica. A, sporophyte consisting of leafy branches giving 

 rise to microsporangia (i), megasporangia (g) and rhizoids (r); B, longitudinal section of 

 portion of branch showing a megasporangium (g) with 3 megaspores in view, a micro- 

 sporangium (i) containing microspores; C, a young microsporangium showing free mother 

 cells before formation of tetrads; D, tetrahedral division of spore mother-cell; E, ripe 

 megaspore; P, four microspores of tetrad separated; G, microsporophyll seen from above 

 showing ripe microsporangium. — ^After Dodel-Port. 



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