Description of the Guayule. 49 



shade plants it is green. The dark-red color extends also over the under 

 surface of the cotyledons, which are rather thick in sun forms, and thin- 

 ner in shade-grown plantlets (plate 34, figs. 4, 6). 



The early foliage leaves, soon after germination and because of the 

 very short internodes, are closely crowded. By partial etiolation these 

 internodes may be caused to lengthen, and thus the structure of the pri- 

 mary epicotyledonary stem may be better studied. In this way points 

 may be made clear which otherwise would with difficulty be explained. 

 The first 8 leaves are usually ovate, entire, slightly acute, and taper 

 suddenly into the petiole (fig. 8). They are clothed, as are all the foliage 

 leaves, with closely set T-shaped hairs (plate 30, figs. 9-11) laid parallel 

 to the axis of the leaf, and thus is produced that light green-gray, satiny 

 sheen which characterizes the plant. The first leaf is usually about i cm. 

 long by 3 mm. wide, though measurements vary a good deal. In the mar- 

 iola seedling the earliest leaves resemble those of the guayule, but differ in 

 being broader and lanose, a difference due to the form of the trichomes, 

 those of mariola being of the "whip " form found frequently in the Com- 

 positae. As the hairs are much thicker on the under side of the leaf, the 

 species may be very readily recognized even when only one foliage leaf 

 has developed, though identification is difficult before this leaf appears. 



The last formed of the entire-margined seedling leaves may reach, 

 in field plants, a length of 7 to 8 cm. and a width of 1.5 cm. The first 

 approach to the mature leaf form is seen in a single tooth, usually on 

 one margin only, at about the middle of the blade. In the next stage 

 the tooth may be found on both sides, and larger, while half-way between 

 their position and the apex a second pair of teeth appears. By basal 

 contraction of the blade and extension of the upper portion, the first 

 teeth appear to move downwards, and by enlarging attain lobate pro- 

 portions. The leaf is now relatively shorter and broader. An additional 

 pair of basal teeth may also add to the complexity. While this descrip- 

 tion, illustrated well in plate 18, is generally true, few plants are more 

 variable as regards the form of the leaf than the guayule, and this varia- 

 bility is, with the exception of the earliest foilage leaves to be formed, 

 closely connected with the amount of available soil-water. Thus we find 

 that in plants grown under irrigation the amount of lobing is very much 

 more marked than in field plants. We shall return to this subject later. 



The first inflorescence is usually formed early in the history of the 

 plant, and may occur in the first growing season even in field plants, 

 though this is exceptional (plate 17). This early flowering in a shrubby 

 plant of long life appears to reflect its relationship to herbaceous forms, 

 and would not improperly be regarded as indicating that the perennial 

 habit of the guayule and mariola is, phylogenetically, a recently acquired 

 character. The inflorescence, which is a compound monochasium (fig. 10) , 

 is terminal, and thus ends the growth of the chief shoot. In some in- 

 stances flowering may not occur for some years, and in this event if no 

 accident befalls the chief shoot it may attain a length of 15 cm. or more 

 before the first flower shoot appears to conclude the growth of the chief 

 axis. In such a case the lateral shoots make but little growth. Upon 

 the first occasion of flowering the growth of the branches begins; these 

 4 



