192 BAILEY AND TUPPER. 



elements in groups I and IV. For example, in groups I and II, 12 

 percent of the specimens are less than 15 years old, whereas in groups 

 III and IV 5 percent are less than 15 years old. 



It is evident, accordingl}', that there is an important correlation 

 between the size of tracheary elements in arborescent and fruticose 

 Dicotyledons and certain types of differentiation of the secondary 

 xjdem. As the vessels become wider and more highly specialized 

 structuall}-, the tracheary elements tend to become shorter. Not 

 only do the elements tend to become shorter, but the bordered pits 

 in their lateral walls become correspondingly modified. Table VI. 



TABLE VI. 



Angiospermae — Dicotyledoneae. 



Tj^pes of lateral pitting. 



Group I Vessel-segments: Scalariform and opposite 86% 



" " " ; Opposite and alternate or alternate 14% 



" Other tracheary cells : Medium or large borders 63% 



" : Small borders 20% 



" " " " : Borders vestigial or absent 17% 



Group II Vessel-segments: Scalariform and opposite . 80% 



" " " : Opposite and alternate or alternate 20% 



" Other tracheary cells: Medium or large borders 35% 



« " " " : Small borders 30% 



" " " " : Borders vestigial or absent 35% 



Group III Vessel-segments: Scalariform and opposite 11% 



" " " : Opposite and alternate or alternate 89% 



" Other tracheary cells: Medium or large borders " 07% 



" : Small borders 33% 



" " " " : Borders vestigial or absent 60% 



Group IV Vessel-segments: Scalariform and opposite 06% 



" " " : Opposite and alternate or alternate 94% 



" Other tracheary cells: Medium or large borders 05% 



" : Small borders 12% 



" " " " : Borders vestigial or absent S37o 



In groups III and IV, alternate-multiseriatc pitting predominates 

 in the lateral walls of the vessels, whereas, in groups I and II, scalari- 

 form and opposite-multiseriate pitting are much in evidence. The 

 tracheary elements which surround the vessels in groups I and II 

 usually have conspicuous bordering areas about the pit orifaces in 



