THE PTERIDOPHYTES. 



201 



In the carboniferous age of the world's history there 

 were many genera, as is evidenced by the remains found 

 in the coal formations, and among these heterosporous 

 forms are known to have 

 existed. 



The stem of Equisetum 

 is subterranean. From 

 it hollow vertical shoots 

 rise ; some sterile, bearing 

 only leaves and branches ; 

 others bearing sporangia, 

 which are located on 

 the inner sides of scales 

 grouped together into 

 cone-shaped tips of the 

 shoots. The ripe spore 

 cases open on the inner 

 side. The spores are 

 greenish in color ; each 

 has four threads, or ela- 

 ters, with knob-shaped 

 ends, which are wound 

 round the spore when 

 moist, but are extended 

 when dry ; in other words 

 they are hygrometric. 

 These threads undoubt- 

 edly help in the trans- 

 portation of the spore. These spores are of great interest 

 under the microscope. The moisture of the breath upon 

 them when dry causes the elaters to fold up. When the 

 moisture evaporates, they again extend themselves. 



The cylindrical shoots are hollow, the woody tissues 

 being arranged in a single circle of fibrovascular bundles. 

 The epidermis contains silica to such an extent that it 



FIG. 129. Equisetum sylvaticum. In 

 the center is a barren branch. On 

 the right and left are fertile branches. 

 (After Bennett and Murray.) 



