THE GREAT GROUPS OF PTERIDOPIIYTES 159 



another leaf branch arises, in which the blade is modified 

 as a sporophyll. In this case the sporophyll incloses the 

 sporangia and becomes hard and nut-like. Another com- 

 mon form is the floating Salvinia (Fig. 134). The chief 

 interest lies in the fact that the water-ferns are heteros- 

 porous. As they are leptosporangiate they are thought 

 to have been derived from the ordinary leptosporangiate 

 Ferns, which are homosporous. 



Three fern groups are thus outlined : (1) homosporous- 

 eusporangiate forms, now almost extinct ; (2) homosporous- 

 leptosporangiate forms, the great overwhelming modern 

 group, not only of Filicales but also of Pteridophytes, well 

 called true Ferns, and thought to be derived from the pre- 

 ceding group ; and (3) heterosporous-leptosporangiate 

 forms, the water-ferns, thought to be derived from the pre- 

 ceding group. 



EQUISETALES (Horsetails or Scouring rushes) 



85. General characters. The twenty-five forms now rep- 

 resenting this great group belong to a single genus (Equise- 

 tum, meaning " horsetail "), but they are but the linger- 

 ing remnants of an abundant flora which lived in the time 

 of the Coal-measures, and helped to form the forest vegeta- 

 tion. The living forms are small and inconspicuous, but 

 very characteristic in appearance. They grow in moist or 

 dry places, sometimes in great abundance (Fig. 135). 



The stem is slender and conspicuously jointed, the joints 

 separating easily ; it is also green and fluted with small 

 longitudinal ridges ; and there is such an abundant deposit 

 of silica in the epidermis that the plants feel rough. This 

 last property suggested its former use in scouring, and its 

 name " scouring rush." At each joint is a sheath of minute 

 leaves, more or less coalesced, the individual leaves some- 

 times being indicated only by minute teeth. This arrange- 

 ment of leaves in a circle about the joint is called the cyclic 



