Foliar Ray in the Wood of the Dicotyledons. 651 



in primitive parts, e. g. the seedling. In passing from these regions to 

 subsequently formed portions of the stem the development of the foliar ray 

 tissue is a progression in the direction of compounding. The significant 

 facts to be kept in mind are that in Quercus the first-formed wood of the 

 seedling possesses solely uniseriate rays, the development of foliar rays 

 is a progression in the direction of compounding, and more primitive stages 

 of the ' linking-up ' process occur in the young plant. The ' linking up of 

 small rays to form large ones in the first rings of the twigs of Quercus 

 Robttr' does not invalidate seedling evidence, since in this species the 

 solidifying of the foliar ray may not have worked back to the region of the 

 pith in the early wood of the stem and branches. 



An unfortunate difficulty has been introduced by Professor Groom : 

 that the broad solid ray may be primitive in oak, since it apparently under- 

 goes subdivision in many cases. In view of the progression in the direction 

 of compounding which occurs in the development of ray structures in the 

 life-history of oak, alder, and other genera of the Fagales it is difficult for 

 the writer to concur in this turning of the foliar compound ray inside out. 

 That the foliar ray is dissected into shallower sheets of ray tissue in passing 

 horizontally from the young to the older portions of a thick stem is true 

 for all species of the Fagales. This is as true for species in which foliar 

 rays are of the aggregating type as for those higher types in which nearly 

 homogeneous masses of fused ray tissue are related to the traces of the 

 leaves. In ' exogenous ' stems which are increasing rapidly in circumference 

 a multiplication of elements, fibres, vessels, parenchyma, and rays must 

 occur with added layers of growth. Therefore the dissection of foliar rays 

 is essential to maintain the proper proportion of ray tissue in the older 

 zones of the stem. However, the impulse which produces this dissection 

 of foliar rays is fundamentally different from that which results in the 

 evolution of storage tissue in relation to the traces of the leaves, and cannot 

 be considered evidence for believing that the broad, solid type of ray is 

 primitive in Quercus. Both phenomena are often active in the same foliar 

 ray. For example, in Alnus rhombifolia, Quercus virginiana, Mill, or 

 Q. densiflora, Hook, and Arn., foliar rays of the aggregate type are gradually 

 ' built up ' in passing from the vicinity of the leaf-trace to the mature 

 portion of a wide stem, yet a dissection of this aggregating mass of tissue 

 takes place in the older wood, resulting in the formation of ' lower ' and 

 more numerous sheets of compounding tissue. 



In this connexion Professor Groom's conclusions in regard to ' Quercus 

 (Pasania) fenestrata (Q. spicatd) ' are of interest : 'In Q. fenestrata these 

 characteristic high rays are often approximated in pairs, and between the 

 two rays forming a pair the boundary of the annual ring is much nearer the 

 centre of the stem than it is elsewhere (PL LXXXV, Fig. 1 2). Moreover, 

 the space between the two rays forming a pair is devoid of vessels. These 



