334 EQUISETALES OR HORSETAILS 



of the cotyledon, which, however, curves upward and spreads 

 out its blade to the light (Fig. 231). Note in Fig. 230 that the 

 necessities of the plant have detern:iined the position of these 

 four organs. The foot is formed in contact with the bulk of the 

 food in the prothallium, the root next the ground which it reaches 

 at once, the leaf easily reaches the light since the root has already 

 ruptured the calyptra, and since the leaf is near the margin of 

 the prothallium, and finally the stem is in a position of advantage 

 (Fig. 230, B) for growing straight up or horizontally. The young 

 sporophyte, which has up to this time drawn its nourishment by 

 means of the foot from the gametophyte, is now in a position to 

 care for itself. The prothallium soon withers, leaving the sporo- 

 phyte an independent and self-supporting plant. The first leaf is 

 small and usually bears little resemblance to those of the mature 

 plant, but as the stem elongates, new leaves are formed which 

 gradually 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 282. 



Order 3. Equisetales. The Horsetails 



114. General Characters. — This small group of Pteridophyta, 

 comprising 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 



