Compound Rays in the Lower Dicotyledons . 239 
the term ‘ medullary’ in this connexion appears to be inadmissible. It has 
been shown that the so-called ‘ primary ’ rays or aggregate rays are in no 
sense related to inclusions of fundamental tissue, but are produced with 
secondary growth by the aggregation and fusion of uniseriate rays, and by 
the parenchymatization of fibres separating these. Similarly the smaller 
multiseriate, bi- and tri-seriate rays, such as occur in Betula, Carpinus , 
Ostrya , and Fagus y are formed by the enlargement of individual uniseriate 
rays or by the dissection of aggregate rays. The primitive type of ray, 
the uniseriate ray, develops only with secondary growth, and is in no sense 
related to the medulla. This becomes very clear from a consideration of 
the comparative anatomy of living and fossil woody plants, particularly 
of the anatomy of plants which retain well developed primary structures. 
Fig. 16 illustrates a cross-section of Lyginodendron oldhamium. It will 
be noted at once that the rays which develop with secondary growth do 
not extend into the primary metaxylem at all, nor are they related in any 
way to the fundamental tissue of the pith. Similarly in living plants which 
retain well developed primary metaxylem, e. g. roots of certain Conifers, 
Larix and Abies , it can be seen that the rays originate only with secondary 
growth. It is only in more highly specialized plants, in which the primary 
structures have been much reduced, that the rays appear to connect with 
the medulla. In consequence it seems desirable to replace the term 
‘ medullary ray ’ by wood ray in designating this parenchymatous tissue 
of the xylem. In the past it has been customary to divide ray structures 
into two classes, the so-called ‘ primary ’ and ‘ secondary rays ’. As has 
been shown above, all wood rays are essentially secondary, in that they 
originate only with secondary growth. However, if these terms are to be 
retained in botanical literature the term primary might well be applied to 
the primitive or uniseriate ray, and the word secondary to the larger rays 
which have been evolved from them. The large ray may be divided into 
two classes according to its mode of origin. The most important large 
rays are the compound, aggregate , or foliar rays whose origin has been 
described in this article. In addition there are certain smaller multiseriate 
rays, usually bi- or tri-seriate, which originate by the increase in size of 
uniseriate rays or by the dissection of aggregate rays. These might well 
be called multiseriate. There is strong evidence for believing that this 
type of ray has been produced, as has the aggregate type, in connexion 
with the leaf. However, this will receive further investigation in this 
laboratory. 
In conclusion, I wish to express my sincere thanks to Professor 
E. C. Jeffrey for valuable assistance and advice in conducting this investi- 
gation. To my colleague in Forestry, Professor J. G. Jack of the Arnold 
Arboretum, I am indebted for much carefully identified green material of 
