SIZE VARIATIOX IX THACIIK AHY CKLLS. 



189 



and shorter cells in small or depauperate plants, and stems which are 

 co^•ered with large persistent branches. However, the size of the 

 tracheary elements varies considerably even in dicotyledons of similar 

 growth forms. These fluctuations appear to be concomitants of 

 certain structural changes in the xyleni. 



It was sliown by von Mohl (1851) in the middle of the last century 

 that Acssels are compound structures which arise from series of cells 

 by the loss of the pit membranes in the division walls between the 

 members of the series. In certain types of secondary xylems, the 

 vessel-segments closely resemble scalariform tracheids in general form 



AGE-ANNUAL RINGS 



Figure 4. Graphs illustrating the effects of environmental factors on the 

 length of tracheary elements. 13. Pimis Taeda, section 1| feet from ground, 

 effects of injury at A; 13a. Same, section 32 feet from ground; 13b. Same, 

 section 62 feet from ground; 14. Pimis contorta var. Murrayana, vigorous 

 specimen; 14a. Same, stunted specimen; 15. Sunetenia Mohogo7ii, libriform 

 fibers, effects of injury at A; 15v. Same, vessel-segments, effects of injury at A. 



and structure. Fig. 6. In these presumably more primitive t^pes of 

 vessels certain of the pits in the end walls of the indi\adual tracheary 

 cells have no pit membranes and are without well marked bordering 

 areas of the secondary walls. These types of vessels grade into others 

 in which the scalariform openings become larger and reduced in num- 

 ber and coalesce to form single large openings or pores. At the same 



