6 5 8 
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., A Inns 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 
