"-^ay in the Wood of the Dicotyledons. 653 



developed as aggregations of uniseriate rays about the persistent leaf-traces 

 of evergreen plants of the warmer Mesozoic times. These sheets of storage 

 tissue were subsequently extended increasingly greater distances from the 

 leaf-traces, and gradually compacted by the coalescing and enlargement of 

 the uniseriate rays and the parenchymatization of fibres included between 

 them. Rays of this primitive type which occur side by side with uniseriate 

 rays have persisted to the present time in certain of the more primitive 

 Dicotyledonous families, e.g. Fagales, Casuarinaceae. 



DIFFUSION OF THE FOLIAR RAY. 



With changes of environment during later geological periods the 

 primitive foliar ray was modified in the evolution of the majority of modern 

 trees and shrubs. This change consisted in the diffusion of the constituent 

 smaller rays of the aggregate sheet of foliar ray tissue. Evidence of this 

 transition, as has been shown by Thompson (12), has persisted in certain 

 Dicotyledons and is particularly well shown in the Casuarinaceae and 

 Ericaceae. In certain species of Casuarina^ e. g. C. equisetifolia, L., 

 C. glauca, Sieber, &c., the typical primitive foliar ray of the aggregate type 

 exists in the younger portion of the stem, but in passing to the older 

 portion the individual rays of the aggregation are diffused throughout the 

 wood. A similar condition exists in Rhododendron punctatum, Andr., 

 Ledum groenlandicum, Retz., and Kalmia angustifolia, L. In somewhat 

 higher types, Fagus atropunicea, Sud., and Platanus occidentalis^ L., the foliar 

 rays have been diffused throughout the wood and evidence of the former 

 congeries has been lost. However, serial transverse and tangential sections 

 show that the diffused parts of the former aggregate ray are related to the 

 leaf-traces. It is to be emphasized in view of the conclusions drawn by 

 Groom from Jost's (8) study of Fagus sylvatica^ L., that the so-called broad, 

 high rays of Fagus represent diffused and enlarged portions of former 

 foliar aggregate rays. In the majority of higher Dicotyledonous trees 

 and shrubs the process of diffusion is so complete that evidence of the 

 former connexion between the multiseriate rays and traces of the leaves 

 has disappeared entirely. 



REDUCTION OF THE FOLIAR RAY. 



In the wood of a large number of Dicotyledonous plants the foliar ray, 

 either in its aggregate, compound, or diffused form, occurs with the primi- 

 tive uniseriate ray. There are, however, numerous exceptions to this 

 general rule. Thus the uniseriate ray is seen to disappear entirely from the 

 wood of certain highly specialized Angiosperms (5). Furthermore, individual 

 species of many families of Dicotyledons are characterized by possessing 

 only uniseriate rays. In PI. LXII, Fig. i is illustrated the tangential section 

 of the mature wood of the common American Chestnut, Castanea dentata^ 



