206 PRACTICAL LESSONS IN SCIENCE. 



any other vein. In other cases the branches of adjacent veins 

 unite forming a net work near the margin of the leaflet. The 

 sporangia in the true ferns are collected in clusters on the back of 

 the frond or are variously arranged in lines along the veins or 

 around the margins. The sporangia may be naked or covered 

 by an indusium. In the case of the ostrich fern, the sensitive and 

 cinnamon ferns there are sterile fronds, and fertile fronds, the 

 latter bearing large globose sporangia, appearing very little like 

 fronds; only a careful study of their life history would make one 

 sure of their origin. 



Besides the ferns, the pteridophyta include the Equisetacese, 

 called horsetails or scouring rushes, and the Lycopodiacese called 

 lycopodium or club-moss. No member of the pteridophyta is of 

 great economic value, yet as the source of our coal supply they 

 have an honorable historical record. 



