IO '92 
Thompson . — The Anatomy and 
ways, first by the enlargement of the uniseriate rays, and secondly, by their 
fusion. The first process, that of enlargement, is itself effected in two ways. 
The cells of a uniseriate ray may simply divide radially to produce a 
biseriate one, which in turn continues the process, or the adjacent tracheides 
may be transformed into ray parenchyma. The latter method is much 
more common, and indeed many rays originate by this transformation 
in the wood and do not extend to the pith. During the process the 
tracheides retain their contents and become septated irregularly. As one 
examines the sections from the pith outwards one finds that the compart- 
ments into which the tracheides are divided become gradually shorter and 
arranged irregularly. Fig. 28, which is a photograph of a tangential 
section near the pith of E. altissima , shows the formation of a number 
of rays in this manner. The ray cells are all very much elongated 
vertically, and some of them are obviously segments of tracheides. Indeed, 
in some places, it is difficult to distinguish the limits of the rays. 
The second method in which the broad rays are formed is the signifi- 
cant process of fusion of small rays. Two or more small rays become 
joined into one large one, while the intervening fibres are either pinched out 
or transformed into ray parenchyma. Later, other rays may be added 
laterally in the same way, so that rays of various sizes are found in the 
compounding area. Fig. 29 shows the compounding process taking place 
in E. monos tacky a. Two of the large rays are obviously in the process of 
formation by a fusion of similar ones. Fig. 30 shows the same thing taking 
place in E. californica. The completion of the process usually requires 
a number of annual rings. Often it is very slow, and the included fibres 
remain intact for a long distance. Indeed, in some cases the fusion appears 
never to become complete, for tangential sections may reveal the presence 
of fibres and even vessels in the rays of very old wood. Such a tangential 
section of A. californica is represented in Fig. 31. The numerous included 
fibres show that this broad ray has resulted from the fusion of smaller ones. 
A similar condition from E. trifurca is shown in Fig. 32. Broad rays such 
as these containing fibres are homologous with the so-called false rays 
of many lower Dicotyledones, e. g. A Inns, Carpinus , Betula. 
The formation of broad rays by the compounding process is exactly 
what Eames 1 has recently described as occurring in the oak. He has 
shown that in the seedling and in the young branch of some species the 
characteristic broad oak ray is not present, but develops later in a manner 
exactly similar to that just outlined for Ephedra . From these facts and 
from fossil evidence he concluded that the broad type of ray found in 
Angiosperms is not a primary structure representing the gap between the 
original primary vascular bundles, as has been supposed, but is really 
derived secondarily from an aggregation of uniseriate rays of the Coniferous 
1 Eames, A. J. : Origin of Broad Ray in Quercus. Bot. Gaz., March, 1910. 
