THE GREAT GROUPS OF PLANTS 245 



notation. The term " Pteridophyta " may still be used 

 to advantage in a more restricted sense, as applying 

 to the "true ferns," while the "fern allies" naturally 

 fall into two other Divisions or phyla, namely the Club- 

 mosses (Lepidophyta) and the Horsetails (Calamophyta 

 of Bessey, or Arthrophyta of Berry). 



The discovery of the fossil seed-bearing ferns (Cycado- 

 filicales) and their fossil and living relatives (Hemicycad- 

 ales, Cycadales, Cordaitales, and Ginkgoales), all having 

 cryptogamic (i.e., centripetal) wood, 1 and all the living 

 forms distinguished from the other gymnosperns by the 

 possession of ciliated motile sperms, suggested the group 

 to which Jeffrey has given the convenient and descriptive 

 term Archigymnosperma (Early Gymnosperms) : in contrast 

 to the Yews (Taxales), Conifers (Pines, Spruces, Hem- 

 locks, Firs, Cypress, etc.), and Gnetales, which lack both 

 those characters. To this latter group Jeffrey has given 

 the name, Metagymnospernuz (Late Gymnosperms). 



Other authors have suggested grouping the woody- 

 stemmed and comparatively small leaved Cordaitales, 

 Ginkgoales, and Coniferales together, and apart from 



1 The first formed woody tissue is primary wood or proloxylem. It is 

 present when the organ (stem, root, etc.) is young, and its cell walls 

 are thickened in rings or spirals and thus it can readily stretch as the 

 organ elongates in growth. After growth in length has ceased, or has 

 been greatly retarded, secondary wood or metaxylem, forms. The cell 

 walls of this tissue have scalariform, reticulate, or pitted thickening, 

 and thus they cannot readily stretch. In the vascular cryptogams (e.g., 

 Club-mosses and related forms) the secondary wood forms inside the 

 zone of primary wood; in the later or "higher" gymnosperms (Metagym- 

 nosperms) this order of development is reversed; while in the ferns and 

 lower gymnosperms (Archigymncsperms) the earlier development is cen- 

 tripetal and the later centrifugal. Thus the mode of development of 

 the woody tissue is an index of the evolutionary position of a^ given 

 form. 



