190 



BAILEY AXD TUPPER. 



time the constituent cells of the \essels become wider and their ends 

 tend to become less tapering. The most specialized condition results 

 in the formation of tubes of relatively large Ijore which are composed 

 of wide segments with nearly horizontal end walls. 



In Table II, four categories of vessels have been recognized. In 

 group I, the perforations are prevailing scalariform; in group II, in- 

 termediate between scalariform and porous; and in groups III and 



AGE-ANNUAL RINGS 

 HEISHT-FEH 



Figure 5. Graph 16. Ps:cudnti^iir)n tnxifnUn, avorage length-on-age curve, 

 for eisht sections removed from different trees and at different heights; 17. 

 iSame, average lengths of tracheids in outer rings of a large stem, at heights of 

 from 2-\M feet above the level of the ground; 17a. Same, Length of tracheids 

 in first annual ring, at heights of from 2-154 feet above the level of the ground. 



IV, prevailing porous Fig. 6. The vessel-segments in group III differ 

 from those in group IV in having well marked tapering ends, thus 

 resend)ling tracheids in general outline. 



That tracheary elements tend to shorten as vessels become more 

 highly differentiated is indicated by the data in Tables II and III. 

 If the material in the various groups were made strictly comparable, 

 it would tend to accentuate the differences between the lengths of the 



