Description of the Guayule. 57 



virgin guayule has been removed, big plants will no more be seen. The 

 largest plants which have been reported weighed in the neighborhood of 

 10 kilograms. Overseers of field experience insist that they have seen 

 and weighed such. Endlich (1905) quotes Marse as having seen a plant 

 weighing 6.5 kilos; but a plant weighing over 5 kilos is exceptional. Of 

 three large plants which are illustrated in this paper, that in plate 12, 

 fig. A, weighed 10.56 pounds (fresh weight), was 75 cm. tall, and 125 cm. 

 wide. Plants over a meter in height are seldom met with, and are nearly 

 always more or less stag-headed end moribund (plate 10). They have 

 usually lost a good many limbs, and for many years have not been mak- 

 ing any net gain in weight. Endlich places the average weight of virgin 

 guayule at 500 or 600 grams. As will develop in the discussion in the 

 following chapter, plants of this size, which would be 40 to 50 cm. or 

 more tall, will in the future be considered large plants. 



SURFACE CHARACTERS OF THE STEM AND METHOD 

 OF DETERMINING AGE. 



The importance and difficulty of determining accurately the age of a 

 particular guayule plant has prompted careful study of the appearance of 

 the surface of the stem at various ages (plate 14, fig. B). This appearance 

 is due to (i) the primary superficial characters (epidermis, leaf-scars) and 

 (2) the succeeding secondary cork. Secondary changes in the cork are 

 produced by weathering. As marks also aiding in the determination of 

 age may be mentioned the dead but persistent peduncles and the number 

 of divarications of the stem, as related to the formation of inflorescences. 

 Data relating to the rate of growth of seedlings, the marks of which are 

 usually quite obliterated in plants taller than 10 or 15 cm., must also be 

 considered. 



FIELD PLANTS. 



Let us suppose that we are examining a plant at the close of the 

 growing season of, say, 1908. The characters seen in the accretions for 

 the years mentioned will be as follows : 



1908. Leaves still adherent. The epidermis is intact and densely 

 clothed with appressed T-shaped hairs, producing the greenish-gray color 

 uniform with the leaves. If the length of the year's growth is exceptional, 

 say above 10 cm., the basal part may show slight longitudinal fissures. 

 Diameter at base 3 mm. or less, rarely more. 



1907. Epidermis still adherent, but more or less fissured, showing 

 yellow cork. The hairs have been partially removed by attrition and 

 withering, but most of them remain, preserving a gray color. Epidermis 

 light brown. Leafless, but scars present. Often with short spurs, or un- 

 developed branches with each a few leaves. Diameter usually between 3 

 and 4 mm. 



1906. Color gray, slightly slaty brownish, generally fissured, the fis- 

 sures shallow, disclosing a gray-colored cork (weathered), with small areas 

 of epidermis remaining between. Diameter about 5 mm. 



1905. The growth for this and earlier years is dark gray, becoming 

 darker with age. The fissures are shallow, becoming deep only with an 

 age of over 10 years. The fissuring is deeper, and lenticels are more abun- 



