April, 1913.] The Classification of Plants, IX. 105 



carpel also shows successive degrees of specialization. The cones 

 and ovuliferous scales of the white pines show an intermediate 

 type of development between those of the spruce and Douglas- 

 fir on the one hand and the more specialized two-leaved pines on 

 the other. 



By some, relationships and phylogenies are interpreted mainly 

 through supposed similarities of the vascular structures. Such 

 classifications are, however, ' vain unless they are supported by 

 the combined evidence of all other structures, at least until it 

 can be shown that the extremely hypothetical assumptions used 

 as a basis for interpretation can be established with some degree 

 of probability. There are no primitive vascular plants known, 

 as indicated above, which might be used as a basis of comparison. 

 The fossil record is a blank for any plants which would lead us to 

 the beginning of vascular evolution and the lowest living 

 Homosporous Pterdophytes show a considerable diversity. 

 The living homosporous classes are about on a general level of 

 evolutionary development and the assumption that the protostele 

 or any other type of vascular structure is the most primitive 

 remains to be proven. There is also no evidence that the vascular 

 system or any other stem structure is less subject to modification 

 than are leaf, root or reproductive structures, none of which have 

 escaped changes of a profound nature. The assumptions based 

 on the embryogeny of the vascular structures are no more certain 

 than those based on the embryogeny of the reproductive parts. 

 Nevertheless, the careful study of the vascular systems will give 

 us another important aid in deciphering the true relationships 

 of the higher plants, provided that the knowledge gained is cor- 

 related with evidence from other lines of investigation. It is, no 

 doubt, pennissable to call supposed embryonic recapitulations 

 to our aid in attempting to reconstruct the hazy course of phylo- 

 genetic history, but it must be regarded as only one of the lines of 

 evidence to be considered along with every other clue one may 

 obtain froin every structure, function, and peculiarity of the plant 

 in its entire life cycle. 



Synopsis of the Cycadophyta. 



I. Leaves compound; stem an unbranched shaft or with few branches. 



1. Megasporophylls only slightly differentiated from the foliage 



leaves; leaves femlike, often very much compounded; no cones 

 formed. (Fossil). Pteridosperm.^. 



2. Megasporophylls highly specialized, usually very different in 



form from the foliage leaves; in Cycas still showing some foliage 

 characteristics; leaves pinnate, rarely bipinnate; at least one 

 kind of sporophylls in cones. CvcADEiE. 



a. Microsporophylls leaf like; flowers probably all bisporangiate. 



(Fossil). BENNETTITALES. 



b. Microsporophylls not leaflike, arranged in compact mono- 



sporangiate cones; diecious. CYC AD ALES. 



