658 Bailey. The Evolutionary History of the 



SUMMARY AND CONCLUSIONS. 



1. The central cylinder of primitive Angiosperms was a tubular 

 cylinder or siphonostele which possessed strongly developed secondary 

 growth. 



2. The wood of the most primitive Angiosperms possessed only uni- 

 seriate or linear rays, such as are a well- developed feature of the wood 

 of Conifers and other Gymnosperms. 



3. During the warmer times of the Mesozoic sheets of storage tissue 

 were ' built up ' from congeries of uniseriate rays, about the persistent leaf- 

 traces of evergreen Angiosperms, and were subsequently extended vertically 

 and horizontally considerable distances from the node. 



4. This primitive type of foliar ray tissue has persisted, in more or 

 less unaltered form, in certain species of primitive families of the Dico- 

 tyledons, e. g. Casuarinaceae, Fagales, &c. 



5. With changes of environment in later geological periods storage 

 conditions were fundamentally modified in the wood of Dicotyledons. 



6. In the evolution of the majority of living Dicotyledonous trees 

 and shrubs the individual units (varying greatly in size owing to the en- 

 largement and fusion of the original uniseriate rays which formed the 

 incipient aggregation) of the aggregating mass of foliar ray tissue have 

 been diffused more or less uniformly throughout the stem, and evidence 

 of their former relation to the traces of the leaves has disappeared, except 

 from certain primitive forms, e. g. Casuarinaceae, Ericaceae, Fagales, and 

 Platanaceae. 



7. In a comparatively limited number of forms the primitive foliar ray 

 of the aggregate type has been progressively compounded or solidified, and 

 has resulted in the formation of foliar rays of the compound type composed 

 of homogeneous masses of parenchyma, e. g. oaks with deciduous foliage, 

 Casuarina Fraseriana, Miq., Alnus rhombifolia, &c. 



8. In many families of Dicotyledons species exist in which a reversion 

 to the primitive uniseriate condition has occurred as a condition of reduction 

 from foliar rays. 



9. Evidence of the reduction of the foliar ray in the Fagales consists of 

 a more or less complete series of progressively reduced species, and of the 

 persistence or recurrence of foliar rays in regions of phylogenetic significance 

 in forms which are very completely reduced. 



10. The importance of experimental plant morphology in the study of 

 phylogeny is clearly illustrated by the Fagales, In its later history the 

 family has suffered vegetative reduction. At the same time storage con- 

 ditions have been fundamentally changed in the family, resulting in modifica- 

 tion of the sheets of aggregated ray tissue which originated about the 



