376 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



come not only the megaphyllous ferns, seed ferns, cycads, conifers, 

 and flowering plants, but also the microphyllous lepidophytes and 

 arthrophytes, while the Bryophyta represent an independent and 

 never thoroughly terrestrial line of evolution from other algal types, 

 and are otherwise unrelated to the higher plants. 



Summarizing the foregoing largely speculative evolutionary sketch, 

 the history of the vegetable kingdom may be divided into a pre- 

 chlorophyllic stage, the first chlorophyllic or algal stage and a second 

 chlorophyllic or pteridophytic stage inaugurated by the beginnings 

 of a land flora. The subsequent evolution of the various types of land 

 plants from fern-like ancestors comprises stages of lesser magnitude 

 and come more completely within the range of observation. 



The most momentous event in the later history of the vegetable 

 kingdom was the first occupation of the land. That we must look to 

 the algae for the origin of land plants is clearly shown by the am- 

 phibious method of fertilization of all the so-called archegoniates. 

 It was probably some of the simpler green algae (Chlorophyceae) 

 that first left the water, somewhat as does the modem Botrydium. 

 The necessity of a water supply resulted in water-absorbing organs 

 (roots), and epidermal cuticularization to limit evaporation, and sub- 

 sequently, to the evolution of mechanical tissue and conduction tissue 

 to enable them to lift their assimilating organs above the surface. At 

 first these primitive land plants were limited to moist environments 

 and have been compared by Campbell with certain liverworts, as 

 those of the class Anthocerotes, which he considers may represent in 

 a general way the stock from which the mosses diverged in one direc- 

 tion and the ferns in another. Paleobotany furnishes no support for 

 this specific view. 



Formation of secondary wood by the various stocks of early land 

 plants was a great advantage and far reaching in its results. Increase 

 in diameter meant ability to reach a greater height and carry more 

 branches, and thus display more foliage, and this meant many changes 

 in habit and greatly increased working power. This may be summed 

 up in greater mechanical strength,'more assimilating tissue, and better 

 conducting tissue. 



Some lines, like the Cordaitean, evolved a regular arrangement with 

 centrifugal additions like that perfected in modern trees. Other lines 

 experimented with the old cryptogamic centripetal secondary wood. 

 Others succeeded for a time in reaching a lofty stature with only a 

 thin zone of secondary wood, relying mainly on the thick cortex for 

 mechanical support. Such were the Lepidodendrons, but that their 

 solution of the problem was incapable of survival by modification into 

 something more efficient is proved by their failure to appreciably 

 survive the Paleozoic. 



