48 



Guayule. 



base. In P. hysterophorus and P. lyratum they are very broad, and are 

 membranous in the former. Figs. 7 and 9 will make these and other 

 characters evident. 



FIG. 9. Achenes of (i) Parthenium argentatum, (2) P. hysterophorus, (3) P. incanum, (4) P. lyratum. 



SEEDLING. 



When germination is complete the seedling of the guayule consists 

 of a short primary stem (hypocotyl), 5 to 10 mm. in length, terminating 

 in a long, slender tap-root. Attempts to find the end of this in the field 

 have been fruitless, on account of the nature of the ground and because 

 of its very tender character and great length and thinness. Experiments 

 show that it reaches a depth of at least several inches. This slender root- 

 let, with very few branches, is the means of keeping the plantlet supplied 

 with water from the soil for some months, as frequently during the first 

 year in the field no adequate development of lateral roots occurs. The 

 seed leaves (cotyledons) are nearly or entirely circular in form, and range 

 in size from 2.5 mm. in width by 3 mm. in length to 4.5 mm. in width 

 and 4.7 mm. in length, according to various conditions. At the apex of 

 each cotyledon is a hydathode, composed of a group of water stomata. 

 Other conditions being the same, seedlings grown in the shade and high 

 relative humidity have the largest cotyledons (plate 34, figs. 6, 9), and 

 the largest were seen on seedlings grown experimentally under such con- 

 ditions. The primary stem is about i mm. in diameter, and in seedlings 

 grown under natural conditions, i.e., with direct sunlight, is dark red; in 



