Entstehung der Tüpfelung. 



13 



verdickt. So entstehen also mehrreihige dünne Wandstellen, welche sich 

 nun schließlich hoftüpfelartig verdicken, daher ihre multiseriate An- 

 ordnuno-. 



ooo 



Spiral- Leiterförmige Netzförmige Ver- 



rerdickung. Verdickung. dickung. 



Penhallow sagt darüber buchstäblich (S. 38): 



„In the Co)iiferales the spirals throughout the entire extent of the 

 protoxylem structure are more or less distiuct, thoiigh there is a more 

 or less definite tendency to coalescence. Such a tendency becomes 

 most pronounced in the lower Gymnosperms, being especially well de- 

 fined in the Cycadaceae and the Cordaitaceae. In the fornier the spirals 

 beconie approximated and blend in such a manner as to deflnitely reduce 

 theareas devoid of secondary growth which theu assume an elongated 

 form and as the latter diminish still further in length, the spirals are 

 eventually replaced by a more general thickening of the wall through 

 secondary growth, and definite pits arise.'' 



-. i 



Längsschnitt eines Teiles des Strobilusstieles von Encei^halartos cycadifoliui 

 nach einer Handzeichuung von Frln. H. L. G. de Bruyn. 



„Such changes are progressive from the protoxylem radially outward 

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

 definite series commeucing internally with typical spiral Clements and 

 terminating outwardly with tj'pically pitted elements, the two being 

 connected by transitional forms. The same structural alterations may 

 be Seen in Cordaites, which oifers a peculiarly iustructive 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 trans- 

 formations which are completed within the transition zone are of 

 great phylogenetic importance as well as of taxouomic interest. it will 

 be necessary to trace them somewhat in detail as they appear in 

 Cordaites Brandlingii. 



In the successive radial development of new tracheids there is a 

 constant tendency to a more uniform thickening of the cellwall by secon- 



