Medullary Rays of Qitercus. 987 
and normal predecessor may occur, and at first sight presents the appear¬ 
ance of being merely an extension of the outer zone of this, as all its vessels 
are small. The question arises in these and other cases as to whether 
these narrow structures are true annual rings formed during an unfavour¬ 
able season, or merely growth-rings representing a 4 doubling ’ of the 
annual ring. It would be interesting if they were autumn- or winter- 
rings in these two species, which grow on mountains in Arizona and 
adjoining regions. 
Class III. Transitional Sub-evergreen and Evergreen. 
A number of sub-evergreen and evergreen species of Quercus possess 
wood whose rings represent transitions from the condition of distinctness to 
that of absence of the pore-zone in the spring-wood. 
Q. chrysolepis , Liebm. (Fig. 3), is an evergreen species, the leaves of 
which live for three or four years. The ring often shows a general gradual 
decrease in the calibre of the vessels from the inner to the outer boundary, 
and in such cases the pore-zone is indicated by wide vessels, which are often 
of the maximum diameter and contrast with the outermost of the imme¬ 
diately preceding ring. Nevertheless, in the inmost zone of the ring narrow 
vessels also occur, while in the middle zone there are vessels of various 
sizes, and the contrast between the inner and the outer halves of the ring 
is not so great as in Class II. The ring is delimited by a layer of 
flattened tracheides. 
In Q. ( Pasania ) densijlora , Hook, et Arn. (Fig. 4), an evergreen species 
whose leaves live for three or four years, and Q. Engelmanni , Greene, 
whose old leaves are all shed as the new ones appear in the spring, the 
ring is at about the same stage as in Q. chrysolepis , though the layer of 
flattened fibro-tracheides is more distinctly marked. But in Q . densijlora 
the somewhat feebly indicated pore-zone is much looser. 
The sub-evergreen Q. virginiana , Mill., has rings marked by a pore- 
zone which is represented by widely separated vessels of maximum size 
contrasting with those of the outer zone of the ring immediately within 
them (Abromeit (’ 84 ) noted the occasional presence of widest vessels in 
the spring-zone). But frequently here and there in the annual ring a vessel 
in the inmost zone is smaller than those farther out in the same radial 
series, so that there is already a tendency for the maximum calibre of 
vessel to be transferred to the middle zone. The layer of flattened 
tracheides is less marked than in Q. chrysolepis . Q. virginiana also pos¬ 
sesses narrow rings whose vessels are all small ; their nature is as unknown 
as in the similar cases of Q. hypoleuca and Q. Emoryi . 
In Q. agrifolia , Nee (Fig. 6), whose old leaves all fall during winter 
and early spring, and Q. tomentella , Engelm (Fig. 7), whose leaves live for 
two or three years, the pore-zone is denoted by a very loose single concen¬ 
tric series of isolated wide vessels, which are often of the maximum calibre 
