On the Origin of the Multiseriate Ray of the 
Dicotyledons . 1 
BY 
W. P. THOMPSON, 
1851 London Exhibition Science Research Scholar , University of Toronto, 
at Harvard University. 
With Plates LXXVII and LXXVIII. 
HE medullary rays of dicotyledonous plants present several distinct 
-L types. The simplest of these is but a single cell wide, and for this 
reason is called a uniseriate ray. It is similar to, and, in part at least, 
homologous with, the usual type of ray found in gymnospermous plants. 
A second type is two or more cells wide, and accordingly is known as 
multiseriate. Such a ray is of widespread occurrence in the Dicotyledons. 
Still another type, recently designated the compound ray, is much broader 
than either of the foregoing, and consists of an extensive homogeneous 
mass of parenchyma. This is the type which gives to the oak wood, for 
example, its characteristic grain. Finally, closely associated with the 
compound ray, one finds in certain woods an aggregation of uni- and 
multiseriate rays, to which the term ‘ false ’ ray has been applied. 
The inter-relationships and evolution of these different types of rays 
have recently formed the subject of a series of investigations in this labora¬ 
tory. Eames 2 has demonstrated from a study of the structure of fossil 
and seedling oaks, as well as from comparative evidence, that the broad 
type of ray has arisen by a progressive fusion of uniseriate rays and an 
accompanying transformation of the included fibres and wood parenchyma 
into ray parenchyma. That is to say, in the ancestral oaks, and in 
primitive regions of modern oaks, only uniseriate rays may be found, and 
these by a process of fusion have given rise to the modern broad oak ray. 
Bailey 3 has emphasized the perfect series of transitions in this process 
furnished by the false rays of the Cupuliferae, and has shown that the 
factor at work in the development of the compound ray has been the 
influence of the leaf-trace. The most natural and convenient place for 
1 Contributions from the Phanerogamic Laboratories of Harvard University, No. 43. 
2 Eames, A. J. : On the Origin of the Broad Ray in Quercus. Bot. Gaz., xlix, March, 1910, 
No. 3, pp. 161-7. 
3 Bailey, I. W.: The Relation of the Leaf-trace to the Formation of Compound Rays in the 
Lower Dicotyledons. Ann. Bot., Jan. 1911, pp. 225-241. 
[Annals of Botany, Vol. XXV. No. C. October, 1911.] 
3 u 
