The Evolution of the Annual Ring and Medullary 
Rays of Quercus. 
BY 
PERCY GROOM, M.A., D.Sc. 
Professor of Technology of Woods and Fibres, Imperial College of Science and Technology , London . 
With Plates LXXIV-LXXVI. 
T HE typical structure of an annual ring of a deciduous oak, as 
represented, for instance, by Quercus sessiliflora , is as follows :— 
The inner part of the ring shows a so-called * pore-zone formed by 
numerous large vessels, which lie in a tissue mainly constituted of tracheides, 
with parenchyma and a few ‘ fibres ’ interspersed among these, and numerous 
medullary rays. 1 
Outside this zone the vessels decrease in calibre, at first suddenly and 
then gradually, attaining a minimum in the outer zone of the summer-wood 
(‘ autumn-wood ’). These smaller vessels are ranged in radial series, and 
are surrounded by rather short, copiously pitted tracheides together with 
some parenchyma. 
At an increasing distance from these vessels the short pitted tracheides 
(‘ parenchyma-tracheides ’) give way, first, to typical elongated tracheides, 
and secondly to elongated fibro-tracheides, whose walls are very thick and 
have scanty little pits. (There may be fibres with very few simple pits 
or devoid of all pits ; but I am not convinced of the correctness of state¬ 
ments to that effect.) Traversing the intervasal masses of tracheides and 
fibro-tracheides are rays, and tangential lines of wood-parenchyma which 
link the circumvasal parenchyma with the ray-parenchyma. The tracheides 
and fibro-tracheides are polygonal or rounded in transverse section, and 
thus may be referred to as ‘ rounded ’. At the outer boundary of the annual 
ring they are replaced by radially shortened fibro-tracheides whose thick 
tangential walls have numerous bordered pits, so that the outermost part 
of the ring is formed of several concentric series of flattened fibro-tracheides. 
Thus there are four distinct kinds of tracheides, which are connected by 
intermediate forms : (i) parenchyma-tracheides ; (2) ordinary tracheides; 
1 For the sake of brevity hereafter the ‘annual ring’ and ‘medullary ray’ will often be referred 
to under the respective names of ‘ ring ’ and ‘ ray ’. 
[Annals of Botany, Vol. XXV. No. C. October, 1911.] 
