ROLE OF THE MICROSCOPE 



185 



That the fluctuations which occur in anatomical structures will 

 ultimately be found to conform in a definite and regular manner to 

 the influences of different combinations of internal and external 

 factors, is indicated by data that have been collected by a number of 

 investigators. 



For example, the work of Sanio,*' Shepard and Bailey," and Miss 

 Gerry,^ and certain data collected by Mr. H. N. Lee at the Canadian 

 Forest Products Laboratory and Mr. W. W. Tupper at the Bussey 

 Institution, indicates that in the development of the stem of arbor- 

 escent Conifers the tracheids increase in size comparatively rapidly 

 for a number of years (see curve A, fig. 24). The duration of this 

 ])eriod, and the rate of increase during the period, vary in different 



12 



C 8 



u. 



o 



25 



100 



50 75 



ANNUAL RINGS 



Fig. 25. 

 Curve showing the variation in the height of the rays in a specimen of red spruce, 



plants and in different species, depending upon the effects of various 

 environmental factors. At the end of this period of comparatively 

 rapid increase in size, the tracheid length may remain constant, but 

 usually fluctuates more or less in succeeding growth rings of the tree. 



•Sanio, Karl. Ueber die Grosse der Holzzellen bei der gemeinen Kiefer {Pinus 

 silvestris L.) Jahrb. Wiss. Bot., Vol. VIII, pp. 401^20, 1872. 



^ Shepard, H. B., and Bailey, I. W. Some observations on the variation in length 

 of coniferous fibers. Proc. Soc. Am. Foresters, Vol. IX, No. 4, 1914. 



* Gerry, Eloise. A comparison of tracheid dimensions in longleaf pine and 

 Douglas fir, with data on the strength and length, mean diameter and thickness of 

 wall of the tracheids. Science, Vol. XLIII, No. 1106, p. 360, 1916. 



