XIH] THE 'PITH' OF HIPPURIS 177 



certainly be regarded as pith, is in reality to be interpreted as 

 xylem parenchyma. He described the occurrence of a number 

 of cauline tracheal elements in the 'pith' region of the embry- 

 onic vascular cylinder near the growing point. These cauline 

 elements were found by Sanio to be ephemeral and impersis- 

 tent; he observed their first appearance at levels above the entry 

 of the first lignified leaf traces. This account appeared to the 

 present writer so singular, that she repeated Sanio's observa- 

 tions in order to see whether the application of microtome 

 methods, by which the history of the tissue in question could 

 be traced element by element, would confirm or refute his 

 conclusions. The result was in all essentials to confirm Sanio's 

 description; the accuracy of his work is indeed remarkable, 

 when it is*cT>nsidered that he was obliged to rely entirely on hand 

 sections for the interpretation of this delicate piece of apical 

 structure. In one stem-apex examined by the present writer, 

 the first cauline xylem element appeared when the stele was only 

 0-08 mm. in diameter (Fig. 115 A^ p. 178). This harmonises 

 with Sanio's statement that in one preparation he observed the 

 first cauline element when the cylinder was about o-i mm. 

 across. The cauline elements gradually increased (Fig. 1 1 5 5) 

 and persisted for a distance of a few millimetres from the apex, 

 becoming gradually less lignified and thinner-walled until they 

 finally disappeared. At the level at which the first lignified leaf 

 trace began to pass in towards the stele (Fig. 1 1 5 5), there were 

 twenty-one cauline tracheal elements. At a slightly lower level, 

 at which the tracheids belonging to eight leaf traces (L) had 

 entered and taken up a position at the periphery of the stele, 

 twenty-one cauline elements could still be identified (Fig. 1 1 5 C). 

 In this particular case, they were found to be just finally vanish- 

 ing at the level at which the seventh set of lignified leaf traces 

 (counting from the apex) entered the stele ; at this level the stele 

 was only 0-2 mm. in diameter. However, a few of the outermost 

 cauline elements were more persistent than the rest, and either 

 themselves became part of the xylem ring, or fused with the 

 leaf traces as they entered. That the lignified elements in the 



