142 THE LIVING CYCADS 



Even after the spore-bearing leaf, or sporophyll, has 

 become modified, the spores may still be uniform in size; 

 but when the spores become differentiated into two sizes, 

 some remaining small while others grow larger, a long 

 step toward the seed habit has been taken. The small 

 spores and large spores are in different sporangia. The 

 small spores, microspores, are male and are compara- 

 tively little modified; the large spores, megaspores, are 

 female and become more and more modified and in the 

 course of evolution pass into the seed condition by such 

 imperceptible gradations that it is impossible to con- 

 struct even an arbitrary definition that would include 

 all seeds and exclude the most advanced megaspores. 

 Plants which have microspores and megaspores are called 

 heterosporous, to distinguish them from their homo- 

 sporous ancestors, in which all the spores were alike. 



An extremely idealistic view, which might pass for 

 a Devonian or Carboniferous heterosporous fern ancestor 

 of the Cycadofilicales, or for one of the Cycadotilicales 

 themselves, is shown in Fig. 79. The large outer leaves 

 are strictly vegetative. Just within the crown of vegeta- 

 tive leaves is a crown of smaller leaves bearing micro- 

 sporangia on the under surface; and in the center is 

 another crown of small leaves bearing megasporangia 

 upon their margins. The inner leaves, bearing mega- 

 sporangia, have become modified, the modification con- 

 sisting in a reduction in size and a simplification of the 

 outline. The microsporophylls, the leaves bearing 

 microsporangia, represent the first stage in the evolution 

 of the cone. The inner leaves, the megasporophylls, 

 show a distinct advance toward a structure which can 

 be recognized as a cone. This is as far as the Paleozoic 



