300 Introduction to Botany. 



The fact that an alternation of sporophyte and gameto- 

 phyte occurs in the life cycle of plants from mosses to 

 flowering plants, that homologous members can be traced, 

 and that there is a definite law of progression, — namely, 

 an increase in the sporophyte and a decrease in the gameto- 

 phyte (compare shaded and unshaded portions of Figs. 

 157, 158, 159), — affords important evidence of a common 

 origin of these different classes of plants, fitting in with 

 geological evidence pointing to the same conclusion. 



HORSETAILS. 



182. Character of Horsetails. — Horsetails, or Equisetums, 

 are interesting not only because of their peculiar form, but 

 because they are the sole representatives of a large and 

 ancient group whose fossils are much in evidence in the 

 coal measures. Like the ferns, they attain in the tropics 

 a much greater size than we are accustomed to see in the 

 temperate zones. The cylindrical, hollow, green stem bears 

 at each node a whorl of leaves, which are united into a tube 

 at their bases. The leaves are much reduced in size and 

 have lost the normal function of leaves, the photosynthetic 

 function being performed by the stems, whose exterior 

 cells are well supplied with chloroplasts, and whose epi- 

 dermis is perforated with numerous stomata. The surface 

 of the stem is encrusted with silica, which gives protection 

 against mechanical injuries. The plants occur in clumps, 

 or large masses, on banks and in low places near streams 

 or pools, their gregarious habit being largely due to running 

 underground stems. 



183. Reproduction of Horsetails. — Asexual spores are 

 produced at the apices of the stems in sporangia which 

 are borne on the under side of umbrella-like leaves, termed 

 sporophylls, aggregated in the form of a cone (Fig. 160). 



