1920] 



LANGDON—DIOON SPINULOSUM 



117 





i 



lowest and last formed continuing a little farther in the direction 

 of the cortex than the one preceding. In this way they constitute 

 a perfectly continuous conducting system for solutions passing out 

 to the cortical strands. These tracheids are especially numerous 

 a short distance behind the pith, just beyond the primary xylem, 

 but passing on through the gap they appear to become grouped 

 (fig. 5), occurring at intervals with great masses of phloem and 

 parehchyma separating the basal parts of each group. Whether 

 this grouping is associated with a seasonal increase in the length 

 of the trace, corresponding to the radial increase and growth rings 

 of the main stele, has not been determined. In addition to the 

 reticulate and scalariform elements constituting these traces, the 

 regular pitted tracheids of the secondary xylem, usually at the ex- 

 treme lateral borders of the gap, are often diverted to one side 

 into a direction more or less parallel to that of the trace, as shown 

 in figs. 3 and 5. 



Worsdell (12) has described in the case of Macrozamia Fraseri 

 a somewhat similar connection between the fibrous strands in the 

 large medullary rays and the fibrovascular elements of the main 

 stele. He states that "a characteristic feature of the radial section 

 of the wood is the large number of outbending strands of tracheids 

 which, passing through the medullary rays, are continuous with 

 the girdle leaf trace of the cortex. " It is worthy of note, however, 

 that these strands of outbending tracheids, or rather inbending 

 in the sense that they are apparently diverted from the direction 

 of the other secondary xylem elements toward that of the rays, 

 are of the pitted type throughout their length, thus homologous 

 with the pitted elements described in the preceding paragraph. 



The peculiar down-curving growth of the scalariform tracheids 

 constituting the foliar strands in the large medullary rays of D. 

 spinulosum is another interesting illustration of the much dis- 

 cussed phenomenon of gliding growth. Vertically these conducting 

 elements may extend merely to the lower borders of the gap and 

 terminate in the irregular bulbous formations illustrated in fig. 4 B, 

 or they may become inserted for a considerable depth between the 

 perpendicular fibrous elements of the main stele (fig. $sc). In 

 their horizontal extent these tracheids may be and probably are 





