Secondary Xylem in Psilotum. 
509 
Fig. 49. 
Fig. 50. 
Fig. 44. Specimen from which numerous sections were cut. e, /, and g , three aerial branches. 
About natural size. Fig. 45. Stele of small branch of rhizome in transverse section. There are only 
three tracheides; sieve- tubes not specially indicated, x 95. Fig. 46. Diarch xylem of stele of 
a rhizome, arrested in development, x 95. Fig. 4 7. Part of a secondary tracheide showing a con- 
striction. x 390. Fig. 48. Scalariform secondary tracheide showing sinuous course. x 95. 
Fig. 49. From a transverse section through the basal part of the aerial branchy, in Fig. 44. a, primary 
tracheides ; b , b, secondary tracheides, incompletely lignified and containing a protoplasmic lining. 
X 390. Fig. 50. Group of four secondary tracheides ( b ), with parts of two primary tracheides (a) 
and adjacent parenchyma. This group is seen at $ , in Fig. 3 in the plate, x 390. 
