SIZE VARIATION IN TRACHPIAKY CELLS. 185 



Length trachnids mm. 



Furthermore, Hartig (1901) and Shepard and Bailey (1914) have 

 shown that the pecuHar compression wood {Rothhoh) which occurs on 

 the under sides of branches and on the concave sides of bent stems is 

 composed of shorter tracheids than the tension wood (Zugholz) which 

 occurs on the opposite sides of the same branches or stems. 



In view of these facts, the longest tracheids might be expected to 

 occur in the old, straight grained wood of the "clear length" of the 

 stem, or that portion between the swollen base and crown which is 

 devoid of large branches. That longer tracheids do actually occur in 

 this portion of the stem, is indicated by the measurements of Sanio 

 (1872), Shepard and Bailey (1914) and Lee and Smith (1916), and is 

 graphically showm in Figs. 4 and 5. Graphs 17 and 17a are of con- 

 siderable interest in this connection. The secondary tracheids near 

 the pith are of similar lengths at distances of from 2-154 feet above the 

 le\-el of the ground. On the other hand, the average length of the 

 tracheids in the older wood — last 60-100 annual rings — is consider- 

 ably less in the 2-26 and 138-154 foot sections of the stem. In other 

 words, it seems probable that as a plant becomes older, the depressing 

 effect of the conditions at the base of the plant extends further and 

 further up the stem, and persistent branches have a progressively 

 greater influence upon the surrounding tissue of the main axis of the 

 plants. Therefore, in certain sections removed from the bases and 

 tops of large trees, it is not uncommon to find that the tracheary ele- 

 ments — after increasing in length for a period of years — tend to 

 become shorter in the outermost rings of the stem; a phenomenon 

 which might easily be mistaken for senility. Graph 5h, Fig. 3. 



