SPIRAL TRACHEIDS -r 



3 



2. Wood parenchyma trachcids. Usually short, cylindrical cells with thin 

 walls, transverse terminal walls, and bordered pits. Confined to the 

 higher Coniferas. 



a. ParencJiyina tracheids. Characteristic of the xylem of the higher 



Conifers, with which their general direction of growth coincides. 

 Found in association with the resin passages. 



b. Ray tracheids. Characterized by their occurrence in the medullary 



rays with which their general direction of growth coincides. 



We may now proceed to a more detailed consideration of the 

 structure and distribution of the tracheids thus classified. 



SPIRAL TRACHEIDS 



The spiral tracheids are so called because of certain narrow 

 bands of secondary or tertiary growth which lie upon the inner 

 face of the cell wall and take the form of definite spirals. These 

 structures are found to present great diversity in the form of 

 their transverse section, which, as shown many years since by 

 De Bary (13, 156), may vary from elliptical or round-rectangular 

 to an almost quadratic form. In general terms they may be de- 

 scribed as ribbonlike and localized thickenings of the cell wall, 

 which are designed for the obvious purpose of strengthening 

 the latter. While this purpose is not always a prominent feature 

 in the Coniferae, it may nevertheless be recognized in the struc- 

 ture of the protoxylem, and it is conspicuously defined in those 

 thin-walled elements to be met with among Pteridophytes or in 

 the higher seed plants, notably in the spiral tracheids of Zea, in 

 which there is a strong disproportion between the thickness of 

 the initial wall and the volume of the cell. In more general 

 terms, therefore, it may be looked for in succulent stems of 

 vigorous growth rather than in those of a more woody character 

 and slow growth ; or it appears more frequently in plants of a 

 primitive type of organization than in those of a more advanced 

 type, in which the elements have experienced a more general 

 growth in thickness, and where, in consequence, special contriv- 

 ances for support are not demanded. From the standpoint of 

 development, therefore, we may consider that the typical spiral 



