﻿FERN ANCESTRY AND ANGIOSPERM ANALOGIES. 24 1 



In this connection, as well as with regard to the theory of the metamorphosis 

 of fern fronds into cycad sporophylls (and stamens), it is of interest to partly outline 

 the theory of change from the ancestral pteridophytes to spermaphytes. 



SPOROPHYTE REDUCTION CORRELATED WITH ELIMINATION OF SEPARATE 

 PROTHALLIAL STAGES AND EVOLUTION OF SEED-BEARING QUASI-FERNS. 



In the ancestral fern lines the nutritive effort of the mature sporophyte, 

 recpiired in the production of myriads of asexual spores, as measured by bulk, was 

 not relatively great. And so long as these asexual spores remained a sufficient or 

 an unchanged reproductive structure, the fertile frond which bore them was fairlv 

 constaut in its external form, being only subject to or altered bv the more obvious 

 evolutionary factors operating within morphologic limits. Such minor variations 

 are, in a measure, illustrated by the altered fertile tips of the fronds of Polystichum 

 acrostichoides or the dimorphic Stritthioptcris germanica, as compared with 

 . Ingiopteris evecta, an ancient type which in full fruit presents a striking sight, its 

 foliar laminae being quite fertile throughout. With the progressive elimination of 

 separate prothallial stages and the incipieucv of megaspore differentiation, from 

 whatever cause, determinable or bathmic, the approximate equilibrium finding its 

 expression in a strong resemblance between the fertile and non-fertile segments, or 

 the fertile and non-fertile fronds of the sporophyte, was disturbed. Concomitantly 

 the resulting megaspore-bearing segment, or frond, became more and more com- 

 plex in organization as it approached more and more nearly the carpellary stage, 

 while the microspore-bearing segment retained a simple structure, or became 

 much reduced and staminate. 



For the further conception of the evolution of carpels and stamens from asexual 

 sporophytes as based on paleobotanical evidence, it matters not whether differen- 

 tiation between the megaspore and microspore segments of the fronds of primitive 

 heterosporous filices was resultant or causative. That is to say, we are in the main 

 concerned most with the manner in which this evolution manifested itself mor- 

 phologically. Presumably, however, differentiation was simply the complementary 

 resultant of the effort or tendency amongst the pteridophytes which hypothetically 

 gave rise to spermaphyte lines, to produce more and more highly differentiated 

 megaspores, these becoming more and more advantageous to the parent plant. 



In the earliest stages of spore differentiation both megaspores and microspores 

 were borne on the same frond. Later a segmentation must have taken place, result- 

 ing possibly in the basal segment of a frond producing megaspores and the apical 

 segment microspores. Secondarily the one segment or the other became barren 

 and greatly altered, or was eliminated. Thirdly, there was a coordinated new 

 grouping of these newly differentiated segments. Fourthly, fugacious fructifications 

 and the most varied stages of bisexnaH/y, moncecism, and dicecism appeared. 



In the case of the living Cycas we may well believe that the sterile apical 

 pinnae of the carpellary leaf once bore microspores. The retention of such abortive 

 parts long after the evolution of a male cone may be due to mechanical or protec- 

 tive value. The organization of the male cone itself on a different branch, with 

 numerous fronds spirally arranged and reduced to staminate scales, constitutes a 



