32 GYMNOSPERMS 



stems, and leaves; but we doubt whether these organs, developed 

 independently, would be so similar that the two groups would be mis- 

 taken for each other. It is easier to believe that the Cycadofilicales 

 have come from the Filicales by direct descent. We believe it to be a 

 fundamental of development that homospory must precede heteros- 

 pory, and that the hcterosporous Pteridophyte condition must pre- 

 cede the seed condition. This tendency toward heterospory is so 

 strong that this condition has been achieved independently in vari- 

 ous hnes of vascular plants. 



The seed is the final stage in the development of heterospory. 

 From what has preceded, it is plain that there might be cases in 

 which it would be difficult or even impossible to draw a line between 

 an hcterosporous Pteridophyte and a seed plant. If the retention of 

 the megaspore makes the sporangium, with its contained megaspore, 

 a seed ; while a sporangium which sheds its megaspore, even at an ad- 

 vanced stage of development, has not yet reached the seed condition, 

 a single individual might be partly fern and partly seed plant. As 

 we have already noted, such a condition actually occurs in Selaginel- 

 la, and may have been rather frequent as the early seed plants were 

 evolving from the hcterosporous Pteridophytes. The two groups, at 

 the transition stage, would be separated by artificial definitions 

 rather than by facts. 



During the period while the advanced mcgasporangium was de- 

 veloping into the true seed stage, it would not be surprising to find 

 the leaf remaining at the fern level. In very recent times, apples have 

 developed so that numerous varieties, of very different aspect, have 

 arisen, while the leaves remained about the same. 



In vascular anatomy, the Cycadofilicales are more advanced than 

 the ferns with which they were associated. To cite a single feature, 

 circular pits are very characteristic of the xylcm of the higher seed 

 plants, while a sclariform marking is equally characteristic of ferns. 

 The known Cycadofilicales have quite generally progressed beyond 

 the sclariform stage, but we should expect to find it in their seed- 

 lings; for even the living cycads pass through the sclariform stage 

 and Slangcria does not pass beyond it, except in certain tissues. This 

 genus also retains a very fernlike leaf. The case of Slangeria is par- 

 ticularly instructive because it was long mistaken for a fern, so that 



