338 Barber . — Cttpressi noxylon vec tense. 
transition from wide cells within to narrow thick-walled ones, 
the latter passing again gradually into wide thin-walled 
elements outside (Fig. 2 ). The outermost band in each series 
usually presents the character of a ‘ true,’ ‘ normal 5 or 
‘ sudden ’ ring, that is a sudden change from flattened summer- 
cells to wide succeeding spring-tracheides. This sudden 
change in the character of the elements is usually regarded 
as correlated with a profound change in the vegetative 
activity of the plant ; and for this and other reasons the 
broad rings of growth described above have been treated in 
the present paper as rings in the ordinary sense of the term. 
It is not to be supposed that these rings of necessity 
represent annual periods of growth, for there are sometimes 
deviations from the above arrangement which would render 
such an interpretation full of difficulty. Thus it occasionally 
happens that the outermost dark band of a series is gradual, 
and more frequently one or more of the inner bands have 
the character of sudden rings. The same band at different 
parts of the section will change from sudden to gradual 
several times over. Lastly, in a few instances, a sudden 
ring appears in the midst of the wide spring-elements, can 
be traced for a short distance, and then completely dis- 
appears (Fig. 4). Taking into consideration this extra- 
ordinary variability, it will be seen that we have no proper 
data regarding the rate of growth of the plant to which 
the specimens belonged, especially as transverse sections 
taken at short distances from one another show great 
differences in the arrangement and composition of the rings 
of growth. The adoption of the term ‘ annual rings’ in the 
present paper is to be regarded as a matter of convenience 
in describing the other characters of the wood. 
This is not the place to review the voluminous literature 
dealing with the effect of climate on the structure of wood x . 
1 For references to the principal papers dealing with this subject see especially 
Biisgen, Bau und Leben unserer Waldbaume, Kap. vii, Jena, 1897; Nordlinger, 
Deutsche Forstbotanik, i, 1874; and Seward, Fossil Plants as Tests of Climate, 
C. v, London, 1892. 
