JontisON— Is Archceopteris a Pteindosperm? 129 



paniculifera, Stur, from the Culm, were arranged into sterile and fertile lobes, 

 Botrychium would be easily derivable from it, and such differentiation or 

 physiological division of labour is to be expected in an ascending series. 



As Archseopteris is one of the earliest land-plants known, its structural 

 features may indicate the possible lines of descent of later-formed groups or 

 of affinity with contemporaneous groups derived in common from ancestral 

 forms. It must be generally agreed that land-plants did not begin in the 

 Devonian epoch, though the earlier rocks reveal little that is definite or 

 reliable, of their presence. The lapse of time, during which the more 

 primitive groups waxed and waned and disappeared, or were replaced by 

 forms better adapted to the changing environment, was, it is stated, as great 

 in the Pre-Carboniferous epoch as that since the Carboniferous records began. 

 Considerable time is all the more a necessity if the view tliat the "Ferns" 

 of the earlier part of the Palaeozoic epoch were mostly Pteridosperms is 

 substantiated. 



It must be remembered, too, that during the Upper Devonian and 

 Carboniferous epochs the conditions were most favourable to luxuriance of 

 plant-life. It is generally agreed that the temperature over the Northern 

 Hemisphere at least, extending from Southern Europe to the Arctic regions 

 (78° N.), was tropical. One authority gives 20°-25° C. as the average 

 temperature. The Ellesmere Land plants (78° N.) are finer and larger than 

 those of Southern Russia of the same epoch, apparently indicating a warmer 

 climate further north. The rain-fall was torrential (greater than any now 

 known, it is stated), and the percentage of carbonic acid in the air was as 

 high as 8. All the conditions were thus at an optimum for vegetative 

 growth. Hence such fronds as those of Archseopteris, 5 feet long, with its 

 thick rachis, are explicable. Under such conditions the pinnatious of a frond 

 are of less systematic value. The luxuriant conditions favoured excessive 

 growth, accompanied by multiplication or repetition of the parts, and thus 

 Archseopteris, with its bipinnate frond and numerous sporangia, may be 

 regarded as the equivalent in Upper Devonian time of a plant built up on 

 the same general plan, but represented by fewer vegetative and fertile organs, 

 in less favourable recent temperate times. From this point of view the gap 

 between Archseopteris and Ophioglossum, looked at externally (the only way 

 available in the ease of Archseopteris), is not so great as appears at first 

 sight. 



Again, the diffuse habit and want of definite demarcation between the 

 vegetative and reproductive regions are signs of a primitive type. The 

 arrangement in Archseopteris meant exposure of a large surface to any 

 adverse local conditions. In the course of time the less elongated, more 



SCIENT, PROC. R.D.S., VOL. XIII., NO. VIII, U 



