182 JOURNAL OF FORESTRY 



supposed to be parallel to the axis. However, as is shown in figures 

 22 and 23, both of these types of orientation occur in Sequoia. 



The presence of tertiary spirals, in the tracheids of both the spring- 

 wood and summerwood, has been considered to be a very characteristic 

 feature of the anatomy of Douglas fir (Pseudotsuga taxifolia (Lam) 

 Britton). However, that this character cannot be depended upon in- 

 variably, as a diagnostic criterion for distinguishing the wood of 

 Douglas fir, is indicated by figures 19 and 20. As is shown in figure 

 19, tertiary spiral thickenings may occur in both the springwood and 

 the summerwood of spruce, and, as is indicated in figure 20, these 

 structures may be absent in the tracheids of certain specimens of 

 Douglas fir. 



SIGNIFICANCE OF THE VARIABILITY OF ANATOMICAL CHARACTERS IN 

 THE CONSTRUCTION AND USE OF ANATOMICAL KEYS 



The preceding examples of the variability of anatomical structures, 

 together with numerous others which cannot be discussed within the 

 limits of this paper, indicate that an unqualified assumption that ana- 

 tomical characters are extremely constant, is not a safe premise in a 

 search for reliable diagnostic criteria. Furthermore, they suggest 

 that the construction of an accurate and efficient system for distinguish- 

 ing the increasingly numerous "timbers of commerce," is not such a 

 simple problem as has generally been supposed. 



There seems to have been an unfortunate tendency among a number 

 of botanists, foresters, and engineers to fail to recognize, or to ignore, 

 the full significance of the fact that the anatomical structure of wood 

 is not always the same in dift'erent parts of a plant, and in different 

 specimens of a species or variety. 



Wood is a tissue which functions in conduction, storage and other 

 physiological activities of plants, and varies, therefore, in different 

 parts of a tree, and in different plants, depending upon the variations 

 in the functions that it is called upon to perform. 



It is evident, accordingly, that a selected anatomical character, in 

 a given group of plants, may be extremely constant or very variable, 

 .depending upon a number of internal (heredity) and external (en- 

 vironment) factors. The inference should not be drawn, however, 

 that all plants growing in similar environments will be structurally 

 similar, since a given physiological effect may be reached by different 

 combinations of anatomical characters. Nor should it be assumed 



