326 BOTANY: PRINCIPLES AND PROBLEMS 



high in air and which has made possible the development of the 

 tall and vigorous plant body with which we are familiar. This 

 advance in external complexity is paralleled by an equally 

 notable internal one, for instead of the relatively simple structure 

 of the moss plant, we find the highly differentiated internal 

 anatomy described in earlier chapters. This is chiefly distin- 

 guished by the development of those tissues for support and 

 conduction which we call fibro-vascular, and which include the 

 wood and the bast. So distinctive of pteridophytes and seed 

 plants is this type of internal structure that these groups are 

 sometimes known collectively as the vascular plants, in distinction 

 from the non-vascular thallophytes and bryophytes. 



The remarkable advance in vegetative structures which the 

 pteridophytes display is not paralleled in their methods of 

 reproduction. The gametophytes form archegonia and antheri- 

 dia, though somewhat smaller and simpler ones than those of the 

 mosses, and motile sperms swim to the archegonia and there 

 effect fertilization. The amount of sterile tissue in the sporo- 

 phyte has, of course, enormously increased, but typical spores are 

 still produced in definite sporangia and scattered abroad just 

 as they are among the mosses. In the higher members of the 

 division, two kinds of spores appear: Microspores, which give rise 

 to antheridium-producing or male gametophytes, and megaspores, 

 which give rise to archegonium-producing or female gameto- 

 phytes. This condition of heterospory foreshadows the evolu- 

 tion of the seed, which distinguishes the last and highest plant 

 group, the seed plants. 



Pteridophytes are not very numerous in species nor do they 

 form a very conspicuous part of the earth's vegetation today 

 except in certain moist and warm regions. A study of fossil 

 plants, however, shows us that members of this division were 

 much more common in past ages, and indeed at certain periods 

 were the most notable element in the plant population. More- 

 over, at that time they included many stout, woody, tree-like 

 species which formed great forests. In competition with seed 

 plants, the group soon fell from its dominant position and the few 

 descendants which it has left to the present day are for the 

 most part reduced and degenerate. 



Three classes are recognized among the existing pteridophytes; 

 the Filicineae or Ferns, the Lycopodineae or Club Mosses and the 

 Equisetineae or Horsetails. These are so different from one 



