94 Guayule. 



The appearance of the stele is now suggestive of a tetrarch structure 

 and is as follows: In a plane at right angles to the primary hadrome 

 plate lie the two leptome bundles, between which and the hadrome plate 

 the primary cambium lies. In a plane coincident with the hadrome plate 

 and just beyond its edges lie two small secondary mestome strands, 

 formed independently. These, which I shall here call the intercalated 

 strands, lie, therefore, in a plane between the two broad primary medul- 

 lary rays (plate 22, fig. 10). The isolated condition of the intercalated 

 bundles is, however, very transient, since the parenchyma rays between 

 the axial hadrome strand and the small intercalated bundles are soon 

 bridged over, the whole, save as mentioned in the following paragraph, 

 coalescing to form a single axial strand of hadrome. Additional second- 

 ary bundles are intercalated between those already present, at first from 

 the four angles of the hadrome wings, so that in a tap-root 1.2 mm. in 

 diameter before me (plate 40, fig. i) there appear 8 bundles, though it 

 must be said that the appearance of the stele in roots of the same size is 

 not by any means uniform. 



The closure of the hadrome wings by meeting the xylem of the inter- 

 calated strands is not complete, and thus are left two islands (analogous 

 to medullary spots) of unlignified cells about the edges of the primary ha- 

 drome plate (plate 22, fig. n). The outlines of these islands are quite 

 irregular, and they may ultimately become compressed or lignified, so 

 that it is only with difficulty that they may be recognized. In thin roots, 

 as especially in the fibrous laterals, the wood cylinder is very compact, 

 and may have no parenchyma rays. In such also the secondary changes 

 in the cortex are less extensive, and the pericycle is much compressed. 



LATER SECONDARY CHANGES: (CORTEX). 



With age the walls of the cortical cells become somewhat thickened 

 and pitted. The intercellular spaces are very regular in shape, and regu- 

 larly disposed. In a tangential section they appear very uniformly len- 

 ticular (plate 28, fig. 5). 



STEREOME AND SECONDARY CANALS. 



Aside from the secondary increase of wood and bast, the appearance 

 of stereome and of secondary canals has to be mentioned. That stereome 

 which appears in connection with primary tissues only may properly be 

 spoken of as primary. Of this there are but two slender bundles (plate 

 23, fig. 6), which consist each of a few (less than a dozen) slender, very 

 thick-walled elements buried in a mass of material derived from the pri- 

 mary leptome by the swelling of the cell-walls till the lumina become 

 indistinguishable. 



The method of origin of these sclerenchyma cells is difficult to deter- 

 mine, and will be discussed beyond. 



The pericambium appears to be continuous, but so far as I am aware 

 the formation of stereome does not involve the cells of this layer. The 

 configuration of the cells of the adjacent secondary cortex is at this time 

 (a root 2 mm. in diameter) very curious, the walls having been distorted, 

 as if by stretching in a radial direction from the primary sclerenchyma as 

 a center. This result would seem, however, to be due to compression by 



