648 Bailey . — The Evolutionary History of the 
and in subsequent growth a recapitulation of the compounding process 
occurs. 
8. Upon the basis of the comparative and developmental anatomy 
of living and fossil plants and the phylogenetic significance of seedlings and 
traumatic regions, the uniseriate ray appears to be the primitive type of ray 
structure in the Fagales, and the large sheets of ray tissue, either of the homo- 
geneous or aggregate type, have developed from them by a process of aggre- 
gation and fusion. 
In subsequent investigations by the writer ( 1 , 3 ) additional evidence 
was secured in regard to the origin and development of these structures and 
their effect upon the stem. 
1. Small twigs of Quercus , Ainas, Carpinus , and Betnla revealed upon 
the removal of the bark a striking and diagrammatic relation between the 
sheets of aggregate or compound tissue and the traces of the leaves. 
2. A study of the development of ray structures in the Fagales made 
by means of transverse and tangential serial sections cut through the seed- 
ling, young and mature twigs, and large stems, showed the leaf to have 
been the stimulating influence in the formation of these large sheets of 
storage tissue. 
3. The latter originated as aggregations of uniseriate rays in the 
immediate vicinity of the leaf-traces, and have been ‘ built up ’ and extended 
vertically and horizontally considerable distances from the node. 
4. The large sheets of aggregate or compound tissue which may be 
called foliar rays , in view of their origin about the entering trace, have an 
important effect upon the development of the stem, since their rate of growth 
is in most cases less rapid than that of the rest of the xylem (PL LXII, 
Fig. 2). This is generally expressed by a strong retarding influence upon the 
growth of adjacent radii of lignified tissue and produces a marked ‘ dipping 
in ’ of the outline of the annual rings in their vicinity (Fig. 2). 
5. The retarding influence of foliar rays is most diagrammatically 
expressed in small mature twigs of oaks with deciduous foliage, in the 
stems of certain highly specialized Angiosperms of vine-like and semi- 
herbaceous habit, and in the fluted trunk of the Blue Beech, Carpinus 
caroliniana , Walt. In the case of the Blue Beech large groups of approxi- 
mated foliar rays of the ‘ false type ’ produce by their retarding effect upon 
growth the large furrows which are a characteristic feature of the bole of 
this tree, the ridges corresponding to the segments in which foliar rays are 
feebly developed or absent. In the mature twigs of oaks with deciduous 
foliage there are strongly developed foliar rays related to the lateral traces of 
the leaves. These lateral leaf-trace rays extend downwards through several 
nodes, and owing to the phyllotaxy of the plant (see PI. LXIII, Fig. 15), 
sheets of storage tissue are formed which are relayed from node to node in 
ten more or less continuous vertical lines along the stem. As is shown in 
