SIZE VAHIATIOX I\ THACIIF.AUY CELLS. 



187 



be considerably shorter than the later formed elements of the secondary 

 xylem. The segments of vessels are usually somewhat shorter in the 

 first formed than the older secondary xylem, although this is not 

 invariably the case, particularly in plants listed as of Group IV. In 

 this group, relatively short vessel-segments may be of the same length 

 in the innermost and outermost parts of the secondary xylem, a 

 phenomenon noted by Sanio (1873-74) in certain Leguminosae. 



AGE-ANNUAL RINGS 



Figure 2. Graphs illustrating variation in length of tracheary elements in 

 passing from innermost to outermost secondary xylem. Lengths of primary 

 elements shown for comparison. 1. Dioon spinulosum; 9. Trochodendron 

 aralioides, vesselless arborescent dicotyledon, stunted specimen; 10. Lirio- 

 dendron tulipifera, fiber-tracheids; lOv. Same, vessel-segments; 11. Carya 

 ovaia, fiber-tracheids after Pritchard and Bailey; llv. Same, vessel-segments 

 after Pritchard and Bailev. 



As is indicated in Figs. 2 and 4» Graphs 10, lOv, 11, llv, 15 and 15v, 

 the tracheary elements in the first formed growth layers of most 

 arborescent and fruticose dicotyledons become progressively longer 

 for a period of from 5-20 years. During succeeding growth in diameter 

 of the stem, the length of the elements remain nearly constant or 

 fluctuate more or less, apparently in response to environmental in- 

 fluences. Injuries or other abnormal growth conditions tend to cause 



