108 Guayule. 



pith cells have thickened, and in the walls of contact the reticulations, due 

 to the broad, ovate, closely-set pits, are very noticeable. The interspaces 

 are large. 



At various lower levels, depending on the time of the year in which 

 the material is taken, will be encountered the young periderm. Ross * 

 speaks of this as beginning very early, and in his material as reaching close 

 to the apex. If a newly grown twig is examined toward the close of the 

 season it will be found that the periderm embraces only a lower zone (of a 

 thickness depending on the rate of growth) at the base of the stem, and its 

 growth involves casting off the leaves which remained on the upper por- 

 tion of the twig of the previous year. This uppermost zone, carrying the 

 overwintered leaves, undergoes some growth with considerable lengthening 

 of the internodes, so that the leaf -scars of the winter bud do not crowd 

 each other as do the bud-scale scars in plants of the temperate regions. 

 The periderm passes upward from this zone, and during the following dry 

 season slowly cuts away the leaves, until by midwinter, earlier or later 

 according to the character of the season, all the leaves of the previous 

 growing season, save the terminal ones, are cast off (plate 14, fig. B). 

 As the periderm extends toward the apex of the twig the epidermis is 

 fissured concurrently, beginning at the base. 



A section near the base of the season's growth shows the following 

 structure: The periderm is three to four cells deep, measuring o.i mm. 

 The xylem bundles measure about 0.5 mm. on the radius, and the pith has 

 a diameter of i mm. Nearly all the bundles are supplied with both corti- 

 cal and medullary stereome. Tracheids are fewer in the outermost zones 

 of the xylem. The primary cortical canals and pith canals have generally 

 enlarged, the largest measuring 0.3 to 0.4 mm. tangentially, with a radial 

 diameter of 0.15 to 0.2 mm. This section has one completed series of sec- 

 ondary cortical canals, and a second row begun. The epidermis is slightly 

 fissured. This amount of growth and secondary change is by no means the 

 maximum possible. The thickest part of the stem of one season's growth 

 of the seedling shown in plate 46, fig. A, had five series of secondary 

 canals, and cork 0.5 mm. thick, the depth of the cortical tissues, primary 

 and secondary, being 2.5 mm. 



A stem of two growth-periods shows the primary and one series of 

 secondary canals, but the two seasons' accretions of wood are reflected in 

 the annular structure of the wood, as in the seedling hypocotyl before 

 mentioned. Here also, therefore, the relation of structure to age is less 

 apparent in the cortex than in the wood cylinder. The whole of the outer 

 leptome (that embraced between the primary and secondary series of 

 canals), is stereomatic; that within the secondary series still retains its 

 sieve character. A considerable thickness of cork has developed. 



Later changes need not be followed year by year, and it will suffice 

 to point out the more important features summarily. The inner periderm 

 normally does not begin until the stem attains a diameter of over 10 mm. 

 (Ross, /. c.), and the primary cortical canals may still be found up to this 

 time or even very much later, e.g., in a stem 28 mm. in diameter, with cor- 

 tex, including bark, 5 mm. thick. The penetration of the inner periderm 



1 His material appears to have been collected in December. 



