i9 2 °] SIFTON—CYCADS 



433 



; with primordial pits from which the secondary pitting has been 



eliminated 



Trabeculae 



These spool-shaped bars, extending in radial series across the 

 lumens of adjacent tracheids, have received considerable notice in 

 literature, owing to the confusion which arose in some cases between 

 them and bars of Sanio. They have not before been figured in the 

 Cycads, but are present, as shown in fig. 8, a radial section of Dioon 

 spinulosum stem. They contain a core or axis composed of a 

 substance which stains in the same way as the middle lamella of 

 the cell. This core pierces the tangential secondary walls of the 

 tracheid and joins up with the middle lamella. These structures 

 are present in higher forms, but their significance is not known. 

 Since they connect with the primary wall, they must have been 

 laid down before the beginning of secondary thickening. 



Summary 



■ 



i. A study of the primary and secondary wood of Cycads 

 indicates the development of reticulate, alternate, and opposite 

 pitting directly from scalariform types. 



2. The grouped, uniseriate, and scattered pitting characteristic 

 of higher forms is shown to be formed by the elimination of pits. 

 In low forms, of which the Cycads are a type, this elimination pro- 

 ceeded without apparent order, forming all types of grouping 



indiscriminately. 



3. Similar arrangements of pits occur in Cordaites, although its 

 type has become more fixed than is the case in the Cycads. 



4. The Cycads, like the Araucarians, have more primitive 

 types of pitting at the ends of tracheids and where they come in 



contact with parenchyma. 



5 . The xylem of certain of the Cycads quite commonly exhibits 



spiral tertiary thickenings. 



6. Bars of Sanio of the Araucarian type are found in both 

 primary and secondary Cycad wood. An elongated type of bar is 

 also present. The Araucarian type is considered the most primi- 

 tive in living seed plants. No explanation of its origin is offered 

 by Bailey's theory. 



7. Trabeculae are present. 



