DEVELOPMENT OF PLANTS 303 



but as the stem elongates, new leaves are formed which gradu- 

 ally become larger and each succeeding one resembles more 

 closely the adult form. The foot disappears with the withering 

 of the gametophyte, and this is soon the fate of the primary root, 

 but numerous secondary roots are formed along the stems as it 

 continues to creep along on or near the surface of the soil. This 

 growth goes on from two to several years before the plant is 

 prepared to develop sporangia upon the leaves, page 254. 



Order 3. Equisetales. The Horsetail Ferns 



114. General Characters. This small group of ferns, com- 

 prising but a single genus of about twenty-five species, is but a 

 remnant of an extensive group of plants that flourished in the 

 coal period and that formed conspicuous features of the vegeta- 

 tion at that time. The structure of these earlier forms has been 

 so perfectly preserved in fossil remains that they give a better 

 idea of the relationship of the group than could be obtained from 

 the living plants. Forms allied to Equisetum doubtless formed 

 during the coal period of the earth large forests and attained a 

 height of sixty to ninety feet and perhaps three feet in diameter. 

 The species that survive to-day are rush-like plants that rarely 

 exceed a foot in height, though a single tropical form supports 

 its delicate stem upon other vegetation and so attains a length 

 of over thirty-five feet. Species of Equisetum are of common 

 occurrence in nearly all countries, living in shallow ponds, 

 swamps, and marshes or drier soils. 



At first sight they show little suggestion of fern relationship 

 (Fig. 232) and impress one as being singularly out of harmony 

 with our common plants, as though they were indeed relics of 

 a past age. The large leaves, which were so characteristic of 

 the Filicales, are reduced to minute papery scales in this order 

 and take little or no part in photosynthesis. These scale leaves 

 are arranged with great regularity at the nodes of the stem, and 

 owing to their close association they grow together, forming a 

 papery sheath with teeth-like points about the stem (Fig. 232, /). 

 It is noteworthy that in allied fossil forms large chlorophyll- 

 bearing leaves occurred. 



