984 Groom .— The Evolution of the Annual Ring and 
(3) rounded fibro-tracheides (‘ fibres ’) with few and small pits ; (4) flattened 
fibro-tracheides. 
The medullary rays exhibit themselves in two extreme forms: (1) The 
midtiseriate rays are thick, high, gradually tapering wedges, which are 
several cells in thickness and many cells in height. (2) The uniseriate rays 
are thin, shallow ribbons, one cell in thickness, but several cells in height. 
Between these extreme forms transitional stages occur in certain parts of 
the wood. 
The wood of other, and especially evergreen, species of Quercus may 
show a widely different type of architecture, so that in some cases the 
annual ring is unrecognizable by the naked eye and all the rays are thin. 
The detailed histology also varies in different species, but this matter will 
not be discussed in the present paper. 
Here I propose to consider only the evolution of the annual ring, 
especially in regard to the vessels and the medullary rays. 
Evolution of the Annual Ring. 
In some evergreen species of Quercus the vessels in the spring-zone 
of the ring show little or no preponderance in size over those elsewhere, 
and are separated by wider tangential intervals: moreover, the band of 
flattened tracheides forming the outer boundary of the ring may become 
obscure or partially obliterated. Thus, in some evergreen species the ring 
is widely different from the typical one of deciduous species ; and in all 
evergreen species the vessels are smaller and the pore-zone less marked 
than those of at least allied deciduous species. 
To ascertain in which direction the evolution of the annual ring has 
moved, it is obviously necessary to know whether the ancestral type of 
Quercus was evergreen or deciduous. Thus the course of evolution may 
have been :— 
1. Regressive from a typical ring with a marked pore-zone. 
2. Divergent from a ring with a feebly marked pore-zone. 
3. Progressive from a ring with no indication of a pore-zone. 
Without subscribing to the opinion that the first has been the direction 
of evolution of the obscure type of ring, I will describe the series of that 
assumption ; this course will have the advantage that the first stage will 
thus be the most familiar. 
The annual ring in Quercus may be identified by the presence of one 
or more of the following structural features :— 
1. A marked ‘pore-zone’ is seen in the inner zone (‘spring-wood’) of 
the ring, because here the vessels are widest. 
2. The vessels near the outer boundary of the ring are of the minimum 
diameter. 
