1T1TKI) TRACHKIDS 



47 



the inner face of the secondary wall there is what is known as the 

 tertiary wall (fig. 3, t.w.}. This is of limited volume in the great 

 majority of cases, and it is to be recognized as a thin layer. It 

 rarely attains any prominence or contributes to the structural 

 variation of the tracheid, but in Taxus, Torreya, and Pseudotsuga 

 it does follow the same course of development as the secondary 

 wall of the primitive tracheids, and develops spirals which are 

 constant and distinctive structural features. Vestiges of such ter- 

 tiary spirals are to be found also in Larix and Pin us, and wherever 

 they occur they are to be interpreted as representing a phase 

 in the degeneration of the spiral structure which has already 

 permanently disappeared from the secondary wall but survives 

 in the tertiary wall to a certain extent. In any transverse sec- 

 tion of a thick-walled tracheid, under favorable conditions, we 

 may observe fine lines disposed concentrically with one another 

 and with the primary wall. These represent the stratification 

 of the secondary wall, and they are substantially due to the 

 process of upbuilding by successive layers. In a longitudinal 

 aspect the wall also shows a series of fine lines cutting the axis 

 of the tracheid and the lines of stratification diagonally, or in 

 a transverse section cutting the latter radially. These repre- 

 sent the lines of striation, due probably to localization of water 

 of organization along definite planes of growth. They are not 

 readily observable under ordinary conditions, but when the cells 

 are macerated in potassium hydrate they stand out with great 

 prominence, and it will be seen in a subsequent chapter that 

 the same effect is also produced by decay. 



The substance of the cell wall consists primarily of cellu- 

 lose, represented by the formula C 6 H 10 O 5 , but in the course of 

 growth alterations arise through the introduction of carbo- 

 hydrate bodies of various kinds, so that the composition can no 

 longer be represented by such a simple formula. Thus in the 

 primary cell wall there is usually a large amount of pectin, which 

 may be differentiated and localized by ruthenium red, which 

 develops a bright cherry or rose-red color and affords a very deli- 

 cate test. In the secondary walls also the cellulose represents 



