THE MAIN LINES OF PLANT EVOLUTION 



FiGUKE 37. PsUopJu/ton. (After Dawson, from Fuller and Tippo, "College Botany,' 

 2nd Ed., Henry Holt & Co., Inc., 1954. ) 



the superficial layers of the plant. The green stem was the main photo- 

 synthetic organ. Sporangia were located at the tips of some of the 

 branches. Such shoots arose not from true roots, but from rhizomes, which 

 were nothing but subterranean stems bearing rhizoids for absorption of 

 water and salts. There were vascular bundles in the stems. Thus Psilophij- 

 ton turns out to be remarkably similar to the hypothetical primitive land 

 colonist of Lignier. 



Current opinion among botanists favors the Psilophytales as ancestral 

 to the remaining three subphyla of vascular plants, not only because the 

 Psilophytales are obviously extremely ancient and primitive vascular 

 plants, but also because different genera show tendencies toward special- 

 ization in the direction of each of these subphyla. Thus some psilophytes, 

 such as Psilophijton and Asteroxylon, had small scale-like leaves which 

 were developed as projections of the superficial layers of the stem. The 



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