346 Causes and Course of Organic Evolution 



Lycopodium to possess as a response to varied environal stimuli 

 — would tend to reappear on the sporophyte generation as 

 it increased in size. On the principle also of growth compen- 

 sation, this vegetative advance might occasionally though 

 rarely be effected in the later history of each individual, through 

 conversion of spore-forming tissue into vegetative tissue. 



There ultimately originated then from the fertilized egg, 

 as an alternating or si)orophyte generation, a leafy body that 

 slowly but surely attained increased size and greater capacity 

 for resistance to subaerial dessicating influences, during the 

 later cambrian, the silurian, and the early devonian periods. 

 The diverse species, genera, and families that branched off 

 from each other in the process culminated in part in the arbor- 

 escent Lepidodendra, Sigillarias, and Ulodendra of the devonian 

 and carboniferous periods. These represent ages of exposure 

 — as their microscopic structure is gradually revealing — to 

 alternating hotter and colder, moister and dryer, brighter 

 and duller, still and windy, atmospheric states, that all con- 

 stituted environal stimuli to which the plants made steady 

 and ever more elaborate proenvironal responses. 



Throughout the lycopodineous group, as in the bryophytic, 

 the leaves tend to be simple, numerous, closely aggregate 

 along the axis; while the axis is usually elongate, resisting, 

 and fibrous. But with our present very limited knowledge 

 of extinct types it is impossible to say whether the lycopods 

 arose from a primitive but common stock with the true mosses, 

 or whether their ancestry is to be sought for in even older and 

 more generalized types. But this in no way militates against 

 their derivation from representatives of green fresh-water 

 algse. 



The small living group of the Psilotacese, that includes only 

 the genera Psilotum and Tmesipteris, suggests by its structure 

 and habits that it is a greatly reduced and condensed offshoot 

 from the main line of the lycopods. The finding of P.illotum 

 over a large jjart of the tropics and subtropics in epiphytic 

 or saprophytic connection, and of Tmesipteris over a wide 

 area of the southern hemisphere, are indications that they 



