58 Guayule. 



dant on the lower surface of stems which are not in a vertical position. 

 This is because of the thicker development of bark on this side. On old 

 stems the fissures attain a depth of a few millimeters and become long. On 

 very old stems the base may become transversely fissured also (plate 10). 



In using the above marks as a means of judging the age of a plant, 

 one may with considerable accuracy judge of the amount of growth for 

 3 or 4 years, and the average of these will come very near to the truth. 

 Some difficulty may be experienced as the result of reduplicated growth 

 in one year confusing the evidence offered by the leaf-scars, which are 

 crowded fairly closely in the region where the internodes of the winter 

 buds occur. These are of the tropical type, there being no specialized 

 scale-leaves, and consist merely of a few terminal leaves of small size 

 which persist till the following season of growth. 



The natural wounding which results in fissures, especially as the stem 

 grows older, as well as the accidental wounding which frequently occurs, 

 usually sets free more or less of the resin, 1 of which large amounts are found 

 in the cortex, as in the pith. The escaping resin collects as drops on the 

 wound and, as it increases in amount, falls on the ground. Under every 

 guayule plant of any size, therefore, a good deal of resin in the form of 

 limpid masses of irregular size may be found. Should it turn out that the 

 resin is of particular value (Chute, 1 909) , as for a special varnish, consider- 

 able amounts could be collected by peons. 



IRRIGATED PLANTS. 



In irrigated plants secondary thickening begins within a short dis- 

 tance (5 to 15 mm.) of the growing-point, and proceeds at a rapid rate. 

 The fissures are very long and straight, and long patches of epidermis 

 are left which may be still visible 30 to 40 cm. from the apex. The color 

 for two years remains a clean, pale yellow, modified by the gray of the 

 adherent hairs wherever patches of epidermis remain (plate 21, fig. A). 

 The diameter, which remains nearly the same throughout the length of 

 a year's growth in a field plant, making the growth cylindrical, increases 

 rapidly in irrigated plants, so that the basal diameter may be three times 

 that of the tip in the first year and eight times at the end of the second 

 year. The early fissuring and the coloring are correlated with this rapid 

 secondary thickening. 



THE LEAVES. 



The leaves of seedlings have already been described. In the adult 

 plant the form of the leaf varies according to the amount of water avail- 

 able and its position on the twig. In general the water-factor determines 

 the amount of lobing. This is apparent in field plants as well as in those 

 grown under irrigation, and the relation is made manifest, in field forms 

 especially, in the sequence of leaf -form seen during the growing and the 

 .subsequent resting period, consequent on drought and cooler tempera- 

 tures. The guayule may be called semi-deciduous, as it sheds a part of 

 the leaves only, namely, those which are produced between the more elon- 



1 Loss of resin by secondary thickening is for the most part prevented by 

 plugging of the resin-canals (Chapter V). 



