238 IOWA ACADEMY OF SCIENCE Vol. XXV, 1918 



iiiciit of the hi.u-lici- animals. It is well woi't;i wiiile. therefore, 

 to point out this characteristic as basic in the evolution of land 

 plants. 



In the lectures of Professor E. C. Jeffrey at Harvard Uni- 

 versity — I am not aware of any publication of the point — the 

 Enibryophyta are next divided into those without traeheary or 

 vascular tissue (Mosses) and those with such tissues. This 

 ag-ain is a distinction of the highest biolo^cal significance. It 

 is the absence of water conducting- apparatus that keeps the 

 mosses of small stature. It was the development of effective 

 water conducting tissues which made it possible for land plants 

 to attain to sizes exceeding those of the humble mosses. These 

 tissues were also necessary for a plant which should continue to 

 vegetate and reach upward in a dry atmosphere, provided only 

 its roots have access to a water supply. Since, therefore, the 

 size, habit and habitat of the principal vegetation of the earth 

 are dependent on the possession of tracheary tissues, it is well 

 to name the higher plant group Tracheata. Owing to these same 

 biclogical conditions, the evolution of these plants has been 

 largely recorded in the structure of their vascular parts. 

 Progress has been made possible by means of ever increasing per- 

 fection and specialization of the water and food conducting 

 organs. Finally, in fossil plants the water conducting cells 

 ?nd vessels have retained their characteristics better than any 

 other tissues, and therefore these cells and vessels offer the 

 fullest chronological record of the chang'es in plant structure 

 throughout geological time. 



The Tracheata were recognized by Jeffrey in 1897 as con- 

 stituting two w^ell marked groups which he named at that time 

 Lycopsida and Pteropsida, on account of their resemblances in 

 certain important features to the lycnpods and ferns respectively. 

 Although \TO may agree with Bower that Lycopodium is the most 

 suggestive plant to consider as ancestral to the entire tracheate 

 j'hyluiii. all the facts point to the conclusion that the fern series 

 is much larger, that it includes the seed plants, and that it was 

 separated from the lycopsidan stock in early or mid Palaeozoic 

 times. 



The fundamental anatomical difference between the lycopods 

 and equiseta on the one hand, and the pteropsida on the other 

 is this. In Lycoj^sida the leaf trace is small, and the water con- 

 (luctiuf cells of the leaf trace, coming into the stem from tlie 



