GEOLOGICAL HISTORY OF GYMNOSPERMS 29 



Both the deposits formed on the surfaces of the continents 

 and those formed along their margins are the first to be destroyed 

 in subsequent cycles of erosion, of which there have been many. 

 Such types of sediments form a relatively small per cent of the 

 geological column. The earliest unmistakable deposits of this 

 sort known are in the Devonian and these do contain the remains 

 of a complex terrestrial flora. In the following pages I have 

 used the term "gymnosperm" in a morphological and not a 

 taxonomic sense for there are serious objections to including in a 

 single phylum all those very diverse plants that agree in possess- 

 ing a micropylar canal. 



PALEOZOIC HISTORY 



Botanical literature abounds with discussions of the origin and 

 mutual relations of the gymnosperms. The older views were 

 based for the most part on the reproductive organs of the exist- 

 ing species while the later interpretations lean heavily on vascu- 

 lar anatomy, and both are highly speculative and have led equally 

 competent students in quite opposite directions. 



We all seem to share to a more or less degree that human ten- 

 dency to consider only certain structures and not the totality of 

 structures. Within my recollection the morphology of the higher 

 plants was simply the morphology of floral parts as it is largely 

 to this day. The monumental system of Engler and Prantl is 

 based almost entirely on the morphology of what might properlj' 

 be called the accessory reproductive organs. There is a de- 

 voutly held tradition among botanists that the reproductive 

 organs are the most conservative of all organs and therefore 

 merit most consideration. A priori I would consider them as 

 more subject to evolutionary change than almost any other part 

 of the plant and much less conservative than, for example, vas- 

 cular structures or even foliar characters. Geologic history, it 

 seems to me, fully confirms this assumption. A few examples 

 will make this clear. Heterospory has arisen independently in 

 nearly all the great groups. Its logical sequel — the seed habit, 

 has also been acquired in unrelated lines, e.g., in the great group 

 of Lepidophytes which culminated in Paleozoic times, Lepido- 



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li*i Ltl/TARY 



