IOOI 
Medullary Rays of Quercus. 
necessarily in favour of the view that in Quercus the direction of evolution 
has been from the broad to the thin rays by disintegration : for it is possible 
to interpret this case in accord with the theory supported by Bailey and 
Eames, by assuming that in Q . fenestrata the thin rays have ceased to 
broaden and heighten at their original rate, so that they do not link up as 
they did in the ancestor, and thus represent a case of arrested development. 
So far, then, there is no crucial evidence deciding whether the primitive 
condition in Quercus was broad-rayed or narrow-rayed. 
Nor can it be said that Bailey’s interesting observations (10 (2)) on 
traumatic oak woods supply decisive evidence. Even if we admit that 
traumatic structure may sometimes be reversionary in essence, Mr. Bailey 
himself points out that severe injury causes solely uniseriate rays to be 
produced at first near the wound, but that slight injuries may cause the 
reverse, namely, a precocious production of broad rays. 
The disintegration of broad rays into narrow ones phylogenetically or 
ontogenetically is conceivably possible by two means. Either the cambium, 
in place of producing ray-tissue, gives rise in certain spots to fibres and 
wood-parenchyma, or fibres and lines of wood-parenchyma invade the rays 
by sliding growth (and this ontogenetically may eventually give way to 
the first method). Either of these hypotheses would equally well explain 
the facts that in the broad rays there occur cells transitional between fibres 
and wood-parenchyma or ray-parenchyma, and that fibres are to be seen 
projecting from the margins into the broad rays. In favour of the invasion 
view may be mentioned the fact that longitudinal wood fibres (belonging to 
the body of the wood) are often bent sharply at right angles to their course 
where they come into contact with a ray, so that each assumes an L shape. 
Summary. 
The annual rings so distinct in deciduous species of Qtiercus become 
less marked in evergreen species. They are to be recognized by one or 
more of the following structural features:— 
(i) Maximum calibre of the vessels in the inner zone of the annual ring; 
(ii) Minimum calibre of the vessels in the outer zone of the ring; 
(iii) Shorter length, nature of the terminal wall, and richer coloured 
contents of the cells of the medullary rays at the boundary of the annual ring ; 
(iv) Changed direction (usually inward dip) of the boundary of annual 
ring where it crosses a thick medullary ray; 
(v) Difference in the number of parenchyma cells forming tangential 
series in the middle and outer zones of the annual ring, and richer coloured 
contents in the outermost zone; 
(vi) One or more layers of flattened fibro-tracheides with abundant 
tangential bordered pitsjorming the outer boundary of the annual ring. 
