402 GENERAL MORPHOLOGY OF PLANTS 



Second, the arrangement, structure and dehiscence of the 

 microsporangia which are very much like the sporangia of some 

 of the lower families of the true ferns. 



Third, the motile sperms. 



Fourth, the form and development of the female pro- 

 thallium with its archegonia which recalls that of Selaginella. 

 In this respect the conifers also resemble the higher Pterido- 

 phytes. 



The progress in evolution made by the Gymnosperms over the 

 Pteridophytes may be briefly stated as follows : 



First. The establishment of heterospory and the permanent 

 division of labor between the two kinds of sporophylls, which 

 was introduced by some of the higher Pteridophytes. This 

 division of labor is even extended further in some species, 

 as in the cycads where the microsporophylls bearing pollen 

 grains (microspores) are on different plants (male) from the 

 macrosporophylls bearing the ovules, which are on female 

 plants. 



Second. The complete division of labor between sporophylls 

 for the production of spores, and vegetative leaves for the chlo- 

 rophyll function. 



Third. The aggregation of the sporophylls into groups, which 

 was also initiated by the higher Pteridophytes. 



Fourth. The more specialized development of tissue systems, 

 the growth of the parts of the sporophyte by a group of 

 meristem cells instead of growth by a single apical cell. 



Fifth. The increasing prominence and importance of the 

 sporophyte and decreasing prominence of the gametophyte. 



Sixth. The lessening in the size of the gametophyte, especially 

 in the reduction of the archegonia, the neck being smaller, and 

 the reduction of the male prothallium so that there are very few 

 or no prothallial cells, while the wall of the antheridium has 

 almost completely disappeared, the tube cell alone remaining, 

 and the number of sperm cells is greatly reduced. (In Mycro- 

 cycas, however, there are sixteen sperm cells, about half the 

 number common in a great many Pteridophytes.) 



