Feb., 1909.] The Classification of Plants, IV. 453 



35. Sphenophylleae. Fossil. 



Paleozoic plants of tree-like aspect and dimensions, with solid 

 jointed stems with a central triarch vascular bundle ; leaves wedge 

 shaped, comparatively small; probably heterosporous, the 

 sporophylls in cones. 



36. Hydropterides. Water-ferns. 75 species. 



Sporophyte with a horizontal rhizome or floating on the sur- 

 face of the water; leaves alternate or whorled; microsporangia 

 and megasporangia borne together enclosed in sporocarps, lep- 

 tosporangiate. Gametophytes developing entirely within the 

 spore walls or protruding only slightly, very short lived; sperma- 

 tozoids large, spirally coiled, multiciliate. 



37. Isoeteae. Quillworts. 60 species. 



Sporophyte with a short tuberous stem with a peculiar type 

 of secondary thickening and with long, erect, grass-like leaves 

 which have a ligule; roots dichotomous; microsporangia and 

 megasporangia large, borne singly, sunken in the expanded bases 

 of the leaves, eusporangiate. Gametophytes very much reduced ; 

 spermatozoids spirally coiled, multiciliate. 



38. Selaginelleae. Selaginellas. 500 species. 



Sporophvte dorsiventral or erect, with monopodial or dicho- 

 tomous branching and dichotomous roots; leaves small, opposite 

 or spirally arranged, ligulate; cells often with a single chloro- 

 plast; sporophylls in bisporangiate cones, the eusporangiate 

 microsporangia and megasporangia single in the axils of the 

 sporophylls. Gametophytes small and short-lived; sperma- 

 tozoids verv minute, biciliate. Some fossil species developed as 

 large trees with secondary thickening by a cortical meristem. 



VI. GYMNOSPERMAE. 500 living species. 



39. Pteridospermae. Fossil. 



Paleozoic seed plants of fern-like aspect; stems short and 

 erect, increasing in thickness, bearing compound leaves. 



40. Cycadeae. Cycads. 90 species. 



Sporophyte with erect, woody, simple or little-branched 

 stems, bearing compound leaves; vascular bundles collateral 

 concentrically arranged, increasing in thickness by their cam- 

 bium; cortical meristem developed in which new bundles are pro- 

 duced; sporophylls in cones, or the carpels sometimes merely 

 in whorls through which the stem grows ; ovule with pollen-cham- 

 ber; female gametophyte becoming large and fleshy; male game- 

 tophyte developing two or more large spirally coiled multiciliate 

 spermatozoids. 



41. Cordaiteae. Fossil. 



Paleozoic branching trees bearing large, long, thick parallel- 

 veined leaves spirally arranged. 



