99 
Wood on Photographic Plates . 
on their walls. This group agrees in producing an image on a photographic 
plate in which the spring wood is negative and the autumn wood positive. 
Pinus Gerardiana 2 is very interesting in this connexion. It has 
haploxylic needles with deciduous leaf-sheaths ; in these features it agrees 
with the second group. On the other hand, it is a three-needled species 
with a central umbo on its cone-scales. The ray-tracheides bear very 
scantily and feebly suggested denticulations ; these features agree with the 
first group. Pinus Gerardiana is, then, intermediate, morphologically, 
between the two groups. The result by this photographic method is 
peculiarly interesting in view of this. The image obtained showed great 
activity all over, with the medullary rays particularly marked in dark lines. 
The general activity is so great that it is somewhat difficult to distinguish 
between the spring and autumn wood, the latter being, if anything, slightly 
more active. This photographic method leaves Pinus Gerardiana occupy- 
ing an intermediate position between the two groups, in agreement with 
its morphological position. 
Conclusions. 
1. The action of wood in darkness on photographic plates does not 
give a suitable method for discovering the incidence of decay before it is 
macroscopically visible. 
2 . Neither does it form a sufficiently critical method of determining 
the amount of moisture present in timber nor of recognizing kiln-dried 
timber or drastically heated timber. 
3. The case for identification of timber by this method is somewhat 
weakened by the fact that different samples of Scotch Pine may give 
reverse results. On the other hand, apart from these few cases of reversal 
in Scotch Pine, it has been found that this photographic method confirms 
the division of the Pines into two groups already differentiated by their 
morphological features. 
In conclusion, my best thanks are due to Professor Percy Groom, M.A., 
D.Sc., at whose suggestion this investigation was undertaken, for the 
material he put at my disposal and for his helpful criticism; also to 
Professor Brereton Baker, F.R.S., for his information on the chemical 
action involved. This research was subsidized by the Department of 
Scientific and Industrial Research. 
I 2 
June , 1921. 
