EVOLUTION OF SEED-PLANTS 121 



(2) The other break in the evolutionary his- 

 tory of Seed-plants is more serious, and the cir- 

 cumstances less hopeful. This is the gap below 

 the Pteridosperms. How were these plants de- 

 rived from Ferns, and from what sort of Ferns 

 did they arise? These are questions which may 

 never be fully solved, for Pteridosperms appear 

 to go back as far as any land-plants of which we 

 have knowledge. Here again the affinities are 

 clear; the habit, anatomical structure, and organ- 

 isation of the pollen-bearing organs establish the 

 closest relations with Ferns. We know of Pter- 

 idosperms which have altogether the anatomical 

 structure of a Fern, and their pollen-sacs are often 

 not to be distinguished from Fern-sporangia. It 

 is when we come to the seeds that our real diffi- 

 culties begin. Those of the Pteridosperms are 

 very highly organised bodies and show very 

 little trace of any relation to the spore-sacs of 

 a Cryptogam. We must here pause to recall to 

 our minds what are the essential differences 

 between reproduction by seeds and reproduction 

 by spores. 



We may take an ordinary Fern as the type 

 of the latter method in its simpler form. The 

 spores, produced in spore-sacs (sporangia) on 

 the frond, are all of one kind; each is a single 

 cell, of rather small size. On germination, 

 the spore gives rise to an independent green 



