The Origin of Land Plants 103 



attain very imposing dimensions, some of the tree- 

 ferns rivaling the palms in size and beauty. The 

 original stem apex often persists, but new leaves 

 and roots are produced to meet the needs of the de- 

 veloping sporophyte, which may live for a century 

 or more. Sooner or later spores are produced, the 

 development following very close that noted for the 

 liverworts and mosses; but in the ferns the spores 

 are borne in special organs or sporangia, which in 

 the simpler types like the adder-tongue ferns (Fig. 

 12, B, C) suggest somewhat the condition found in 

 Anthoceros. 



The curious horsetails or scouring rushes com- 

 mon in low ground and moist thickets represent a 

 second class of pteridophytes which differ strikingly 

 in their habits from the ferns. The hollow- jointed 

 stems with the leaves reduced to toothed sheaths en- 

 circling the joints, together with the characteristic 

 cones which bear the spore cases, at once distinguish 

 the horsetails from the other pteridophytes (Fig. 13, 

 C, D). The elongated, pendent sacs, or sporangia, 

 containing the spores, are arranged about the mar- 

 gins of the umbrella-shaped " sporangiophores " 

 which make up the terminal cones. In their struc- 

 ture and development the sporangia are not essen- 

 tially different from those of the lower ferns. 



About twenty-five species, all belonging to the 

 genus Equisetum, are all that are known to exist 

 at the present day, the insignificant remnant of the 

 many large and complex horsetails that flourished in 



