XXXIIl] MESOXYLON 271 



including leaf-bases, reaches a diameter of 5 or 6 cm. ; the large 

 pith consists in the central region of diaphragms of parenchyma 

 separated by horizontal spaces produced by splitting and shrinkage 

 consequent on the failure of the tissue to keep pace with the 

 general growth of the stem. The secondary xylem is of the 

 Araucarian type and has narrow medullary rays varying in 

 depth from 1 to 25 cells. The leaf -traces are represented by twin- 

 bundles which fuse in the downward direction, the level at which 

 fusion occurs being regarded as a specific character. The presence 

 of centripetal xylem is an essential feature of the traces : the 

 occurrence of single or double traces consisting of centripetal 

 elements and, externally, a large amount of centrifugal xylem 

 is an important feature in which Mesoxylon differs from Cordaites. 

 The double leaf-traces divide after emerging from the secondary 

 wood and each strap-like leaf receives several collateral bundles 

 (fig. 483, C). An axillary bud may occur at the base of each 

 leaf (fig. 483, A, b). The phloem, including sieve-tubes and 

 secretory sacs, is succeeded by a broad pericycle, and the com- 

 paratively narrow cortex is traversed by successive bands of 

 periderm. In the outer region of the cortex the presence of 

 radial bands of fibres is a characteristic feature. The reproductive 

 organs are unknown. The anatomical features are well illustrated 

 by M. Sutcliffii first described by Scott, who provisionally placed 

 it in Poroxylon, and afterwards more fully investigated by Maslen 1 . 



Mesoxylon Sutcliffii Scott. 



The average diameter of the stem is 3 cm. : the section repro- 

 duced in fig. 483, A, has a maximum breadth of 3-5 cm. ; the 

 leaf-bases cut at different levels give an irregular contour to the 

 surface like that of a Lepidodendron. An axillary bud, either 

 reproductive or vegetative, is seen at b consisting of a short axis 

 bearing crowded bud-scales. The leaves are crowded and according 

 to Maslen have a phyllotaxis of ^- : the lamina is linear like that 

 of Cordaites with 16 collateral bundles in the petiole. The presence 

 of a meristematic band at the base of the lamina affords evidence 

 of a deciduous habit. The large size of the pith is a striking 

 feature with its central tissues in the form of transverse diaphragms 



1 Maslen (11). 



