, • 



1920] WRIGHT— PIT-CLOSING MEMBRANE 239 



condition, pitting of the bordered scalariform type. With this 

 stain the primary wall shows broad and black through the sec- 

 ondary, dividing the narrow red borders of adjacent pits. This is 

 most apparent in the upper half of the tracheid to the left, where 

 the scalariform openings are uniseriate, extending from side to 

 side of the tracheid. In the lower half of the tracheid the primary 

 wall has not been cut. The pit borders are more or less clear, 

 also, about the middle of the tracheid to the right where the pits 

 are small, oval, and biseriate. A combination of silver nitrate 

 solution and ammonia, used with a counter stain of methylene 

 blue, demarked these borders most clearly, but, unfortunately, did 

 not lend itself to photography. 



On the other hand, the metaxylem of Helminthostachys and 

 the metaxylem and secondary wood of Botrychium exhibit a much 

 greater differentiation. The tracheids, as seen in longitudinal 

 section, are irregular and frequently nodular in appearance, with 

 pitting distributed equally on their radial and tangential walls. 

 The section illustrated in fig. 14 is from the rhizome of B. obliquum, 

 cut tangentially and stained with haematoxylin and safranin. The 

 tracheids are irregular in size and position, and interspersed with 

 uniseriate medullary rays. The central tracheid shows the typi- 

 cal pitting of the secondary wall. The uniseriate and biseriate 

 pits are large, round to oval in shape, with a centrally placed 

 round pore. The small shaded area surrounding the pore is ligni- 

 fied. 1 In the tracheids to right and left is depicted a feature 

 characteristic of both Botrychium and Helminthostachys, a ter- 

 tiary wall of lignin. About the center of the tracheid to the left 

 this layer appears as reticulately arranged bars lying over the 

 pitted secondary wall. Above the center the plane of section is 

 lower, exposing only the secondary wall; below the center it is 

 through the lumen of the tracheid, and consequently the tertiary 

 layer is seen in section. In the tracheid to the right, both the 

 tertiary and secondary walls have been cut only in section. Fig. 1 5 , 

 also from B. obliquum, gives a sectional view of the pits with their 

 overlaid scalariform. The pit cavities are approximately twice as 



'In all the text figures lignification has been indicated by means of shading, 

 and a different focus or an obscure feature by dotted lines. 



