SUMMARY AND CONCLUSION 293 



zoids within this, which are discharged, with the 

 contained water, into the cavity above the archegonium, 

 and fertilize the latter in the same way as among the 

 Pteridophytes. 



Comparing the homologies of the higher Pteridophytes 

 and the flowering plants, we find that both produce two 

 sorts of sporangia, macrosporangia and microsporangia, 

 known usually among the latter group as ovules and 

 pollen-sacs. In the latter, spores develop precisely as 

 in all the Archegoniates from the lowest to the highest, 

 i. e. by the division of each sporogenous cell into four 

 spores. The macrosporangium, or ovule, of the Sper- 

 matophytes generally contains but a single macrospore, 

 or embryo-sac, although there are some exceptions to 

 this rule. Very often one or both of the preliminary 

 divisions in the sporogenous cell are suppressed. The 

 sporangia of the Spermatophytes are usually borne upon 

 sporophylls — carpels or stamens — which are the homo- 

 logues of the sporophylls of the Pteridophytes. 



Of the Spermatophytes, the Gymnosperms are obvi- 

 ously the lowest types, i. e. they show more clearly their 

 derivation from the Pteridophytes. Their more primi- 

 tive character is borne out both by a study of their struct- 

 ure and by their geological history. It is not likely 

 that all the Gymnosperms constitute a homogeneous 

 class. It is much more probable that they represent 

 the remnants of two, and possibly more, quite distinct 

 developmental lines. The Cycads show close affinity 

 with the true ferns, while the Conifers recall more 

 strongly the Lycopods. Both of these groups, espe- 

 cially the Cycads, are much less abundant at the present 

 time than in earlier periods of the earth's history. 



