186 



BAILEY AND TUPPER. 



It is evident, accordingly, that the means in Table I cannot be con- 

 sidered o priori, as the tracheary size norms of the species listed. How- 

 ever, in view of the varied character of the material studied, the table 

 as a whole may be considered to give a fairly reliable indication of the 

 general size (length) variations that occur in the tracheary elements of 

 the stems of Coniferae. Long tracheids tend to occur in the " clear 

 lengths" of the stems of tall, rapidly growing forest trees; short 

 tracheids in xerophytes, small, slowly growing or depauperate indi- 

 viduals, and plants whose stems are clothed nearly to the level of the 



AGE-ANNUAL RINGS 



Figure 1. Graphs illustrating variation in length of tracheids in passing 

 from the innermost to the outermost secondary xylem. Lengths of primary 

 tracheids shown for coini)arisr)n. 1. Dioon ,spinidosu»); 2. Araitcnrin Bid- 

 ivillii; 3. Pscwiotsuga laxifolia, after Lee and Smith; 4. Pinus palustris; 

 after Shepard and Bailey; 5. Pinus ponderosa, 6. Cedrus Libani; 7. Taxus 

 brevi folia; 8. Pinus albicaidis. 



ground, with large persistent branches. In Table III, the proportion 

 of the former types of growth forms is higher in the Taxodieae and 

 Abieteae than in the Taxaceae and Cupresseae. 



It has been shown in Table II that, in the stems of dicotyledons, 

 the first formed tracheids, fiber-tracheids and libriform fibers tend to 



