468 BOTANICAL GAZETTE [june 



Fig. 20. — Cycadeoidea Dartoni: radial longitudinal section of secondary 

 xylem, showing transitions between scalariform and opposite and alternate 

 pitting; Xioo. 



Fig. 21. — Juglans nigra Linn.: radial longitudinal section through cam- 

 bium (left) and young phloem (right), showing relation between sieve plates 

 and primary pit areas; Xi8o. 



Fig. 22. — Kayea paniculata Merrill: radial longitudinal section of second- 

 ary xylem, showing primary pit areas and "bars of Sanio"; X200. 



PLATE XV 



Fig. 23. — Taxodium distichum (Linn.) Richard: radial facet of tracheid, 

 showing scalariform structure of primary wall and crowded bordered pits; 

 X400. 



Fig. 24. — Larix occidentalis Nutt.: radial facets of 3 tracheids, showing 

 scalariform structure of primary wall; X350. 



Fig. 25. — Taxodium distichum: radial facet of tracheid, showing scalari- 

 form structure of primary wall and less crowded bordered pits than fig. 22,] 



X400- 



Fig. 26. — Larix occidentalis: radial facets of 2 tracheids, showing numerous 



primary pit areas having no superimposed bordered pits; X220. 



Fig. 27. — Pinus Strobus: radial facets of 3 tracheids, showing modified 

 t3T)es of primary wall structure ; X 220. 



Fig. 28. — Pinus Strobus: radial facets of 3 tracheids, showing reduction 

 in bordered pitting and persistence of primary pit areas; X220. 



Fig. 29. — Larix occidentalis: radial facet of tracheid, showing curving and 

 fusing (or widening) of bandlike thickenings of middle lamella; X240. 



Fig. 30. — Pinus Strobus: radial facets of several tracheids, showing 

 scalariform structure of primary wall; X220. 



Fig. 31. — Taxodium distichum; radial facet of tracheid, showing curving 

 of bandlike thickenings of middle lamella with reduction in number of bordered 

 pits; X400. 



Figs. 14, 17, 18, 20, and 21 were made from sections loaned to the writer 

 by Drs. G. R. Wieland and L. H. MacDaniels. 



