114 Guayule. 



rather regularly repeated periclinal division, and the tissue, therefore, has 

 much the appearance of a cambium. It may also happen that repeated 

 divisions occur in a zone about one of the medullary canals. The cause of 

 this is not clear, though it is possible that this also is a mode of growth of 

 the pith (plate 42, fig. 5). It does not appear to be the same as the forma- 

 tion of cork, such as I have observed to occur following injury to the pith 

 or adjacent tissues. 



In field plants normally neither pith nor parenchyma rays (save a 

 very few cells) ever become lignified. 



The wood in large stems shows the usual distinction of alburnum and 

 duramen. The latter is reddish-brown in color, and all the tracheids are 

 plugged by " Gummipropfen. " 1 Temme (1885) and Ross (1908) note 

 their positive reaction to phloroglucin, which I have verified. They are 

 very sharply confined to the duramen in uninjured stems. In one, 2.5 cm. 

 in diameter, in which the plugs are beginning to be formed with irregular- 

 ity, their genesis may be followed. They first appear as a thin, partial or 

 complete lining, increasing irregularly and gradually filling the lumen. 

 Their conformation suggests the behavior of a dense fluid. Their positive 

 reaction to aniline blue, which is very marked, may indicate that they are 

 at first similar to callus, but, as phloroglucin shows, they later become 

 lignified. In the old wood the plugs appear homogeneous, but they stain 

 unevenly with, e.g., Bismarck brown. Here and there one may note a 

 stratification in planes parallel to the surface of the lumen. That resins 

 are absent from these structures is shown by their total failure to react to 

 alkanet. Molisch (Zimmerman-Humphrey, 1893) showed that gum-plugs 

 behave, with certain reagents, like lignified membranes, but a total par- 

 allelism is denied by the above reactions. Lignification in any event 

 would appear to be a secondary feature. Tschirch (1906, p. 1180) identi- 

 fies the substance as "bassorin." 



ANNULAR STRUCTURE. 



The mature wood shows to the naked eye an annular structure which 

 is frequently regarded as annual-ring structure. In an old stem what is 

 seen in part is a banded appearance due to differences in color intensity 

 (plate 2, fig. B) , having no relation at all to a true annular structure, which 

 is readily seen under magnification. This is shown in the two figures, one 

 of which (plate 33, fig. i) was drawn to scale from the inner alburnum of 

 a very old stem, and the other (plate 33, fig. 2) from one a centimeter in 

 diameter, showing ten rings. It is not at all unlikely that these rings 

 represent ten years' growth, but this would not justify the conclusion that 

 the rings are always correlated with age in years. It must not infrequently 

 be the case that more than two accretions of growth occur in response to 

 the distribution, in time, of the rainfall, and these rings, therefore, repre- 

 sent periods of growth following rain. That these growth-periods for field 

 plants usually coincide with the summer seasons follows from the general 



1 "Wound-gum" (Temme, 1885) seems hardly a suitable term, since the 

 phenomenon is perfectly normal, though, as will appear, the earlier secretion is 

 provoked by natural and by artificial wounding. A direct translation of Gummi- 

 propfen would be preferable. 



