CHAPTER XXII 



THE ARCHIGYMNOSPERMAE: CYCADOFILICALES 

 AND CYCADALES 



The earlier seed plants of Paleozoic and Mesozoic time were 

 characterized by the possession of naked seeds upon which the 

 microspores or pollen grains were directly deposited. The free 

 exposure of the seminal structures to the air has gained for the 

 types marked by this feature the appellation of gymnosperms. 

 In the large group thus characterized there are two main modifi- 

 cations. In the earlier and more primitive gymnosperms the 

 pollen grains or microspores were accommodated in a chamber in 

 the apex of the megasporangium known as the pollen chamber. 

 This cavity, rilled with fluid derived either from the surrounding 

 parenchymatous cells or in some cases from a special fibrovascular 

 system present in the walls of the megasporangium, provided 

 for the germination of the microspores and the subsequent fertil- 

 ization, effected in every instance by swimming sperms. These 

 forms in general present a marked resemblance to the Filicales, 

 often in external habit and always in internal anatomical structure. 

 In contrast to them are the higher gymnosperms, in which there 

 is no true pollen chamber. Here the act of fertilization is effected, 

 not by swimming sperms, but by means of a pollen tube developed 

 as an outgrowth of the microspore and directed toward the egg. 

 The external habit of these gymnosperms is never fernlike, and 

 their anatomical organization shows only the slightest cryptogamic 

 features in living forms. The types possessing pollen chamber, 

 swimming sperms, frequent filicinean habit, and cryptogamic 

 organization of the fibrovascular structures are conveniently 

 designated as the Archigymnospermae or more primitive gymno- 

 sperms. Those forms which are contrasted to these by the absence 

 of pollen chamber and the presence of pollen tube in connection 

 with fertilization, as well as by the disappearance of filicinean 

 habit and anatomical characteristics, may be, on the other hand, 

 designated as the Metagymnospermae or higher gymnosperms. 



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