228 Bailey . — The Relation of the Leaf- trace to the Formation of 
rays is most easily accounted for on the basis of the inclusion in them of 
what was originally wood parenchyma. In the Live Oaks the stages of the 
process by which the crystal-bearing wood parenchyma cells are fused into 
the compound or aggregate rays may be seen clearly. Furthermore, as in 
the case of alder, seedling plants afford interesting evidence in regard to the 
phylogenetic history of the development of ray structures in oak. Eames 1 
has pointed out that seedling White Oaks possess in their early wood only 
the primitive type of ray, the uniseriate ray. In subsequently formed layers 
the large homogeneous ray of the mature wood is formed by an aggregation 
and fusion of small rays and included fibre and wood parenchyma cells. The 
seedling Black Oaks display similar progressive stages of compounding (see 
Fig. 1 5), but in these oaks the aggregating tissue extends back to the earliest 
formed wood, indicating that the aggregate ray is more firmly established 
in the plant in this sub-genus. In contrast to this, in the early formed wood 
of the mature twigs of the Live Oak, there exist conditions similar to those 
which occur in seedling White Oaks, except that the aggregate ray develops 
further from the pith, and passes through the early stages of compounding 
less rapidly. Fig. 26 illustrates a twig of Live Oak in transverse section. 
The four aggregate rays at the top of the figure can be seen to originate in 
the central part of the section, and from the radius of curvature of the rings 
it can be seen that several years’ growth intervene between the pith and 
the point where the aggregations originate. 
Additional evidence of the formation of the large rays in oak by 
a compounding process has been pointed out by the writer in an article 
cited below. 2 In this article it is shown that the wounded wood of 
mature oaks may revert to primitive stages of compounding in the ray. 
Since, in recent years, it has become an established principle that traumatic 
areas may be the seat of reversion to primitive characters, the study of 
wounded regions in oak supplies valuable evidence in regard to the phylo- 
genetic development of ray structure. Rays immediately external to the 
traumatic areas are uniseriate, but in passing out from the wounded area 
(see Fig. 14) there occur all the steps by which the small rays are aggregated 
and transformed into a homogeneous mass of ray parenchyma. 
From the palaeobotanical evidence afforded by Miocene oaks with ‘ false 
rays ’, taken into consideration with that afforded by seedling plants, trau- 
matic areas, and the distribution and development of compound rays in 
several genera, we see that primitive oaks and alders possessed only 
uniseriate rays. With the necessity for a large food reserve and storage 
system, due to the development of unequal seasonal temperature and rainfall 
1 Eames, A. J. : On the Origin of the Broad Ray in Quercus. Bot. Gaz., xlix, March, 1910, 
No. 3, pp. 161-7. 
2 Bailey, I. W. : Reversionary Characters of Traumatic Oak Wood. Botanical Gazette, 
November, 1910. 
