CLASSIFICATION OF VASCULAR PLANTS 65 



so popular in botanical nomenclature, the words Embryophyta 

 , and Thallophyta readily suggest one another. 



The division of Embryophyta into those groups without 

 tracheary cells and those with such organs may be opposed 

 because it involves a negative character. On the other hand, 

 the Vascular plants have long been recognized as a group with 

 profound natural affinities. This distinction, used by De Can- 

 dolle over a hundred years ago, was powerfully reasserted by de 

 Bary's anatomical treatment of the group. It is rendered per- 

 manent and unequivocal by the critical researches and logical 

 analysis of Professor Jeffrey and his school. There is nowhere 

 a more "natural" unit than the Tracheata and the tracheary 

 tissues which have been used to prove this fact may well be 

 referred to in naming the group. The term Vasculares, referring 

 to vessels, is not so apt, seeing that true vessels are absent in a 

 large portion of the group. 



The most brilliant achievement of inductive reasoning since 

 the days of Hofmeister was the recognition by Jeffrey in 1898 

 of the two great phyla of tracheate plants, the Lycopsida and 

 Pteropsida. This distinction has been freshly vindicated by 

 every discovery bearing on the problem. The fundamental dif- 

 ferences in stem, leaf, and sporangium between these two phyla 

 are absolutely basic to any discussion of the broader aspects of 

 comparative anatomy of vascular plants. 



It will be readily recalled that in all Lycopods and Equiseta 

 the leaves are very small (microphyllous) . The sporangia of 

 Lycopodium, Selaginella and Isoetes are clearly related to the 

 upper surface of the leaf (adaxial) . In Equisetum the sporangio- 

 phores are the relics of an axillary or dorsal outgrowth of the 

 sporophyll. The sporophyll has entirely disappeared from living 

 Equiseta. The leaf trace of Equisetum is small, and its separa- 

 tion from the vascular system of the stem makes no gap or 

 other modification in the latter. A similar condition exists in 

 Lycopodiales. But in those Lycopodiales with a hollow cylin- 

 der of vascular tissues, this cylinder is interrupted or broken 

 open by the passage of a segment of it, entire, out into a branch 

 (cladosiphonic). 



