653 
Foliar Ray in the Wood of the Dicotyledons. 
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. glaucci, 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 atropnnicea , Sud., and Plcitanus occidentalism 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. LX 1 1, Fig. i is illustrated the tangential section 
of the mature wood of the common American Chestnut. Castanea dentata , 
