PTERIDOPHYTA 319 



The Gametophyte 



The microspores produce an exceedingly reduced gametophyte (Fig. 282, C). 

 A small sterile cell is first cut off, and the body of the spore then divides further, 

 so that four sperm-cells, surrounded by as many sterile cells, which are finally 

 broken down, result. The sperinatozoids are multiciliate like those of the Ferns. 



The development of the female gametophyte (Fig. 283) is much like that of 

 Selaginella, but does not begin until the spores are set free. About fifty free 

 nuclei are formed before any cell-divisions occur. The archegonia are much 

 larger than those of Selaginella, and resemble more nearly those of the euspo- 

 rangiate Ferns, with which the embryo (Fig. 284) also shows some points in 

 common, notably the absence of a suspensor. In the origin of the stem-apex of 

 the embryo, Isoetes resembles also some of the Monocotyledons. 



Fossil Pteridophytes 



On comparing the fossil Pteridophytes with their living descend- 

 ants, it is clear that the proportion of species in the different groups 

 has changed very much. During the Palaeozoic age, the Ferns were 

 almost entirely eusporangiate types, the Marattiaceae being especially 

 well represented. The Leptosporangiatae do not become at all 

 prominent until the Mesozoic, from which time they increase rapidly 

 in importance, uiitil they have now very largely supplanted the more 

 primitive Eusporangiatae. 



The other two classes, Equisetales and Lycopodiales, have not 

 succeeded so well in adjusting themselves to modern conditions, and 

 these classes, especially the former, are but degenerate remnants of 

 once much more important types. The more highly specialized 

 arborescent forms, like Calamites and Lepidodendron, have entirely 

 disappeared, and it is the much simpler types that have persisted. 

 Whether Lepidodendron may have given- rise to seed-bearing de- 

 scendants is a disputed question. 



It is evident that some of the fossil Pteridophytes are not readily 

 assignable to any of the existing classes. This is notably the case 

 with the Sphenophy Hales, a group which in its anatomical structure 

 seems to partake of the character of both Equisetales and Lycopo- 

 diales. It is possible that the Psilotaceae may be remotely related 

 to the members of this class. Another important class of extinct 

 plants are the Cycado-filices, which were intermediate in their 

 characters between Ferns and Cycads. 



BIBLIOGRAPHY 



'96. 1. Arnoldi, W. Die Entwickelung des weiblichen Vorkeims bei den 

 heterosporen Lycopodiaceen. Bot. Zeit., LIV. 1896. 



'92. 2. Atkinson, G. F. The Study of the Biology of Ferns. London and 

 New York, 1892. 



