1919] BAILEY— BARS OF SANIO 



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younger to the older metaxylem or secondary wood, there were 

 transitions betweerl typical scalariform and opposite and alternate 

 multiseriate pitting. Such transitional stages between scalariform 

 -and multiseriate pitting have been observed in a number of 

 Sphenophyllales, Calamariales, CycadofiHcales, Cordaitales, and 

 Bennettitales. In Protopitys, Cycadeoidea Dartoni (Coulter and 

 Chamberlain) Wiel. (figs. 14, 20), and other forms whose secondary 

 jcylems show indications of zonation, such transitions occur periodi- 

 <:ally in the older wood of the stem, as they do in the stems and 

 roots of the vesselless angiosperms, Tetracentron and Trochodendron 

 (figs. 15,16). In these transitional regions the elongated primary pit 

 ^reas and Querleisten that underlie the scalariform secondary walls 

 tend to persist in tracheids having horizontal rows of bordering areas. 



It is such transitional types of tracheary pitting that have been 

 figured by Jeffrey in the cone axes of Araucarians, and by Sifton 

 in the petioles of Cycas. In Araucaria Bidwillii Hook., owing to 

 the fact that the middle lamella is often relatively thick and the 

 pits not closely approximated, the Querleisten are frequently broad 

 and conspicuous. In transitional tracheids they may break into 

 fragments which cling to the margins of the bordered pits, even 

 after the latter have shifted to the alternate arrangement. Eventu- 

 ally, however, the more or less circular primary pit areas of the older 

 tracheids appear to become surrounded on all sides by equally 

 thickened portions of the middle lamella. 



Scalariform and transitional types of bordered pitting occur in 

 the lateral walls of the vessels of many dicotyledons. In fig. 3 is 

 illustrated the typical scalariform bordered pitting that occurs in 

 the radial and tangential walls of the vessels of certain Magnoliineae. 

 This type of pitting is in marked contrast to the multiseriate pitting 

 shown in figs. 4 and 9. As these figures indicate, the bordered pits 

 which form the horizontal rows may be closely packed together 

 and have flattened sides, or they may be more loosely arranged and 

 have oval outhnes. Transitional stages, between these typical 

 scalariform and multiseriate types of pitting, are of frequent 

 occurrence in the vessels of certain of the Magnoliineae (figs. 4, 11). 



In photomicrographs of carefully stained sections there are 

 thin dark colored lines between the elongated or scalariform 



