354 G wynne- Vaughan. — On ‘ Mixed Pith ' in Osmunda regalis. 
may be safely taken as strong confirmatory evidence in the consideration of 
phylogenetic hypotheses. 
My thanks are due to Dr. R. Kidston for kindly taking the photographs 
used in illustration. 
Summary. 
1. An anomalous stem of Osmunda regalis is described, the pith of 
which at certain points contains scattered medullary tracheae. 
2. It is held that this gives support to the theory that the pith of the 
Osmundaceae is phylogenetically stelar and not cortical, and that it arose by 
the progressive conversion of the central tracheae of a solid xylem strand 
into parenchyma. 
EXPLANATION OF PLATE XXI. 
Fig. i is from a drawing. The others are from untouched photographs. 
Fig. i. Osmunda regalis. Transverse section from the lower end of the specimen, showing the 
mixed pith and the greater part of the xylem ring, x 52. 
Fig. 2. Osmunda regalis. Photograph of another section from the same region, x 50. 
Fig. 3. Osmunda regalis. Surface view of the wall of one of the medullary tracheae to show 
porose pitting, x 200. 
Fig. 4. Osmundites Kolbei. Surface view of the wall of one of the medullary tracheae, x 200. 
Fig. 5. Osmunda regalis. Transverse section of the upper anomalous region, showing the 
traumatic cracks and a few medullary tracheae, x 50. 
Fig. 6. Osmundites Kolbei. Longitudinal section through the stele, showing the elongation of 
the tracheae in the 1 mixed pith ’. x i o. 
