XXVIIl] 



ANATOMICAL FEATURES 



29 



in their distribution. The wood of Stangeria is peculiar in con- 

 sisting of scalariform tracheids 1 (fig. 397). Chamberlain describes 

 growth-rings in the wood of Dioon ; but this is exceptional. In 

 tangential sections of the stele leaf-trace bundles are constantly 

 seen passing horizontally through the broad and deep medullary 

 rays. The pith-cast of a cycadean stem reproduced in fig. 398 

 shows the wide meshes in the reticulum of tracheal tissue originally 

 occupied by parenchyma, which on decay left lenticular depressions 



FIG. 397. Tracheids from the stem of 

 Stangeria paradoxa. (After Marsh. ) 



FIG. 398. Pith-cast of a Macrozamia 

 stem, (f nat. size.) 



represented on the cast by tapered convex areas occasionally 

 bearing the impress of an outgoing trace in the form of a narrow 

 groove. The secondary phloem often rivals the xylem in breadth 

 and is not always easily distinguishable from it; it consists of 

 sieve-tubes, parenchyma, and fibres. The secondary cambial 

 cylinders characteristic of Cycas, Encephalartos, Macrozamia, and 

 Bowenia, to which reference was made in the summary of 

 anatomical features, arise in the pericycle, and a few layers of 



1 Pavolini (09); Marsh (14). 



