22 DESCRirTlA'E CATALOGUE 



often be traced, up to sturdy complex trees of large eize and 

 high differentiation of tissues. All living trees belong to this 

 group, and also the great majority of the forms important in 

 the recent floras. There are two main cohorts, one with naked 

 seeds (the Gymnosperms), the other with seeds enclosed in an 

 ovary (the Angiosperms). 



Class GYMNOSPEEM53. 



Plant-body never much reduced, the majority of the forms 

 being trees or sturdy t-hrubs with much secondary wood. Seeds 

 tend to be large, all are naked, attached to open scales and not 

 enclosed in an ovary. Seed-bearing scales may or may not be 

 associated and modified to form cones. Fertilisation by means 

 of passive nuclei brought by a pollen -tube or active fn-e- 

 swimming spermatozoa. Embryo multioellular, developed 

 within the endosperm, with two or more cotyledons. 



Sub-class CYCADOPHYTA. 



This sub-class includes all those forms, both living and fossil, 

 which have short trunks, with the characteristic Ct/cad-like 

 external features. The sub-class is essentially one based on 

 the external and vegetative morphology, for most of the fossil 

 representatives have fructifications profoundly different from 

 the simpler fructifications of the living genera. The plant- 

 body of all the Cycadophyta, however, is extremely charac- 

 teristic, and unlike that of any other cohort, so the foundation 

 of the comprehensive group by Prof. Nathorst (1902) seems well 

 justified. 



The essential external characters of the plants are as fol- 

 lows : trunk short, in some cases underground, in the majority 

 of cases not more than a few feet high ; if branched the branch- 

 ing is irregular and adventitious, and the branch repeats the 

 characters of the main trunk (see Stopes 1910, Wieland 1906, 

 etc.). The whole outer surface of the trunk is covered by the 

 persistent leaf-bases, which have extended rhomboidal outlines 

 and are in very close spiral series. The leaves are all large 

 and compound, typically with a strong central rachis and simple 



