140 



EVOLUTION OF PLANTS 



except for the conspicuous lobes referred to above. 

 The reproductive organs are very much lilce tliose of 

 the eusporangiate ferns, and the spermatozoids, which 

 are large and multiciliate, closely resemble those of 

 Osmunda. 



Fig. 36 (Equisetineae) . — A, upper part of a sporiferous shoot of a horse- 

 tail (Equisetum pratense), showing the division into nodes and inter- 

 nodes, the rudimentary sheath-leaves, sh, and the strobilus or cone of 

 sporophylls, c; B, a cross-section of an internode of E. maximum, 

 shovriug the arrangement of the vascular bundles, v, and the air-spaces, 

 or lacunfe, I ; C, longitudinal section of the apex of a young shoot of E. 

 maxinmm, sliowing the single large apical cell, a; ; D, a single sporophyll 

 of the same species with the sac-shaped sporangia, sp ; E, median sec- 

 tion of the sporophyll ; F, a ripe spore, with the elaters, el. 



The sporophyte, however, shows many points of dif- 

 ference which are early manifest. Thus, in the embryo, 

 it is the stem-quadrant which grows most actively, 

 while the development of the leaves is subordinated to 

 it, as it is throughout the life of the sporophyte. In- 

 stead of the short stem and large leaves of the ferns, 

 the stem in Equisetum is very much elongated, while the 

 leaves are reduced to the toothed sheaths which sur- 



