THE EVOLUTION OF THE SPOROPHYTE. 13 



between the typical leptosporangiate ferns and the type found 

 in Botrychium, although it must be admitted that the affinities 

 of Osmunda are perhaps quite as evidently with another 

 ancient group of ferns, the Marattiaceae, an order of tropical 

 ferns which are closely related to many extinct carboniferous 

 ferns. 



It is probable, although this cannot be absolutely proved, 

 that such forms as Ophioglossum and Osmunda, which have 

 the whole leaf, or leaf segment, covered with sporangia, are 



FIG. 5. a, The upper part of a leaf of an adder-tongue fern (Ophioglossum), showing the spike 

 of simple sporangia, b, Diagram of a longitudinal section of a part of the spike, showing 

 the cavities containing the spores, c, Sporangial spike of Botrychium simplex, with the 

 few large sessile sporangia, d, Part of the sporangial spike of B, Virginianum, with 

 numerous small, somewhat stalked sporangia. 



more primitive than those in which the sporangia are borne 

 upon the backs of ordinary leaves. 



Wherever the sporangia are borne upon specially modified 

 leaves (sporophylls) we have structures which may be legiti- 

 mately compared to the flowers of the seed-bearing or " flower- 

 ing" plants, where the flower consists essentially of similar 

 modified spore-bearing leaves, or sporophylls. 



In passing from the lower to the higher Pteridophytes, there 

 is manifested in several places a remarkable phenomenon 

 which is of the greatest importance as bearing upon the origin 

 of the seed-bearing plants. This is known as " heterospory," 

 or the production of two sorts of spores. 



With the increasing importance of the sporophyte in the 

 Pteridophytes, there is an accompanying reduction in the 



