-8 ANATOMY OF THE GYMNOSPERMS 



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In the Coniferales the spirals throughout the entire extent 

 "f the protoxylem structure are more or less distinct, though 

 there is a more or less definite tendency to coalescence. Such a 

 tendency becomes most pronounced in the lower Gymnosperms, 

 being especially well denned in the Cycadaceae and the Corda- 

 itaceae. In the former the spirals become approximated and 

 blend in such a manner as to definitely reduce the areas devoid 

 of secondary growth, which then assume an elongated form ; and 

 as this latter diminishes still further in length, the spirals are 

 eventually replaced by a more general thickening of the wall 

 through secondary growth, and definite pits arise. Such changes 

 are progressive from the protoxylem radially outward through 

 the entire extent of the secondary wood, so that there is a defi- 

 nite series commencing internally with typically spiral elements 

 and terminating outwardly with typically pitted elements, the 

 two being connected by transitional forms. The same structural 

 alterations may be seen in Cordaites, which offers a peculiarly 

 instructive illustration of the process because of the regularity 

 with which the changes arise and the extent of the structure in 

 which they lie. As these transformations which are completed 

 within the transition zone are of great phylogenetic importance 

 as well as of taxonomic interest, it will be necessary to trace 

 them somewhat in detail as they appear in Cordaites Brandlingii. 



In the protoxylem region the structure is wholly composed of 

 spiral tracheids (plate 3). In the successive radial development 

 of new tracheids there is a constant tendency to a more uni- 

 form thickening of the cell wall by a secondary growth. This at 

 first finds expression in the more compact arrangement of the 

 spirals (plate 4), which later coalesce at various points, thus giving 

 rise to more localized areas devoid of secondary thickening, and 

 hence to a scalariform structure in which the general lines con- 

 form more or less closely to the direction of the original spirals 

 (plate 5). By a further modification the elongated, thin areas 

 become converted into shorter, often isodiametric areas substan- 

 tially by a process of division. A further tendency to general 

 thickening of the walls causes the margins of the scalariform 



