430 GYMNOSPERMS 



on this basis, would stand not at the beginning but near the end of 

 the line. 



In the gymnosperms, also, the arrangement will depend upon the 

 structure chosen as a basis. 



In the cycad, Macrozamia, there is a group of species closely re- 

 sembling M. spiralis. It would be dubious to arrange these species 

 in linear order, each species derived from the one before it. Probably 

 some of the species have come, independently, from the highly 

 variable M. spiralis. However, if two of the species differed some- 

 what from M. spiralis, but resembled each other in the structure of 

 the gland and venation of the leaf, it would seem more probable that 

 one of them had come from the other than that both had developed, 

 independently, identical minor features. 



Even the open carpel, from which the gymnosperms are named, is 

 not more universally present than the free nuclear period at the 

 germination of the megaspore. This free nuclear period is more 

 characteristic of living gymnosperms than the single cotyledon is of 

 monocots, or the two cotyledons are of dicots; but it would be hard 

 to defend a claim that it is due to inheritance. It is a natural conse- 

 quence of heterospory. Homosporous forms do not have any free 

 nuclear period, and, as heterospory was beginning to emerge from 

 homospory, there may have been no free nuclear period. Some living 

 heterosporous pteridophytes have no free nuclear period, but with 

 heterospory in such an advanced stage as in Sclaginclla and I socles, 

 the early nuclear figures are too small to segment the comparatively 

 large mass of protoplasm, and a free nuclear stage results. After 

 a free nuclear stage has been established, it might persist, even in 

 small megaspores, as in Taxus. 



A free nuclear stage in the embryo of gymnosperms, immediately 

 following fertilization, occurs in all gymnosperms except Sequoia and, 

 perhaps, Gnclum. It resembles that which occurs at the germination 

 of the megaspore, and occurs for the same reason — the early nuclear 

 figures are too small to segment the comparatively large mass of 

 cytoplasm. The free nuclear periods in the young gametophyte and 

 embryo do not indicate relationships. They are merely similar re- 

 sponses to similar conditions. 



The seed is the final stage in the evolution of heterospory. There 



