THE WOODY DICOTYLEDONS 381 



in earlier pages, the organization includes only tracheary elements. 

 It is thus apparent that vessels, a characteristic feature of structure 

 in dicotyledonous woods, may in certain instances be absent. 

 There is no reason based on the general principles of comparative 

 anatomy for regarding their absence as a primitive feature. In 

 woods of temperate climates the vessels in the spring wood are 

 often much larger than in the later growth, and the organization 

 in such cases is described as ring-porous. The ring-porous condi- 

 tion is not, however, universal in trees of higher latitudes and is 

 not ordinarily found in tropical woods which in general have their 

 annual zones indistinctly marked. 



The tracheary elements in dicotyledonous wood, as has been 

 indicated previously, undergo very numerous modifications. In 

 the lower forms they resemble the similar structures hi the gym- 

 nosperms, but quite generally they lose to a large extent their 

 water-conducting function and become mechanical or storage 

 elements. The least advanced condition in the mechanical direc- 

 tion is designated the fiber- tracheid, distinguished by the re- 

 duction in size and decrease in number of the bordered pits as 

 well as by increase in length and by thickening of the walls. The 

 libriform fiber, a further modification, has lost or nearly lost the 

 bordered pits, these being replaced by simple pores. In the sub- 

 stitute fiber, which retains its protoplasmic contents, and in the 

 septate fiber, which is divided by delicate partitions of pectic cel- 

 lulose into a number of separate units, are seen storage modifica- 

 tions of the tracheids present in the higher types of dicotyledonous 

 woods. 



The parenchymatous elements of dicotyledons are primitively 

 scattered throughout the annual ring. Although rarely and only 

 under experimental conditions revealing by actual transitions 

 derivation from tracheids, the storage parenchyma is generally 

 grouped in longitudinal fusiform or pointed series with robust 

 and lignified partitions which clearly indicate its origin. In the 

 systematically higher dicotyledons the parenchyma is character- 

 istically grouped in clusters around the vessels. This condition is 

 known as vasicentric and is a common feature of woods in which the 

 tracheary elements have become partially or entirely mechanical. 



