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 Qtiercus, Alnus, Carpinus, and Betula 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 PL 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 



