﻿28 Curtis: The Evolution of 



further by the growth of the fertile branch into a paniculate 

 cluster of lateral appendages, in Danaea this expansion assumes 

 a leaf form with the synangia covering its entire under surface and 

 in Angiopteris we see the separation of the synangia into soral 

 groups of sporangia. In the entire class of Filicinae the com- 

 plete separation of the sporogenous and assimilatory function does 

 not seem to be perfected, and in many of those species, where 

 the propagative tissue is confined to special leaves, the ease with 

 which they may, in many cases, be transformed to organs of as- 

 similation would seem to indicate the transmission of an inherited 

 tendency towards the association of these two functions. 



The Equisetinae and Lycopodinae show only the remotest 

 alliance with the Filicinae. In the former class the separation of 

 the sporogenous and assimilation tissues has become complete, 

 and this is true to a large degree among the Lycopods. In both 

 groups the aggregation of the archesporial tissue about a sterile 

 tissue, as shown in the fertile spike, is suggestive of the apical por- 

 tion of a sporogonium and the development of organs to bear the 

 sporangia recalls the trend of morphological variation, noted among 

 the Filicinae. 



This separation of function reappears essentially unaltered 

 among the Gymnospermae and Angiospermae and the distribution 

 of labor has now become so thoroughly relegated to certain organs 

 as to allow the leaves a wider range of morphological variation 

 than was possible when associated with the propagative function 

 when the tendency to variation might have so seriously interfered 

 with the propagative function as to have jeopardized the perpetu- 

 ation of the species. The ancestral characters are still preserved 

 in the cone and spike with their prolific spore formation, but with 

 the advantages accruing from the aggregating of the staminate 

 and carpellary sporophyls on a single axis as well as from the wide 

 range of morphological variation that appertained to both the leaf 

 and sporophyl there appears a conspicuous reduction of sporoge- 

 nous tissue in approaching the highest order. Whatever view may 

 be held as to the development of the leaf from the sporogonium, 

 certainly it must be admitted that the sporophyls have not re- 

 sulted from a metamorphosis of leaves and that essentially the 

 reverse process has attended the evolution of plant forms. Re- 



