118 



Guayule. 



by a half-circle. To the right of the vertical diameter is shown the irri- 

 gated plant; to the left of it the field plant. The upper quadrants show 

 these when wet ; the lower, when dry. To be noted are the greater capacity 

 of the wood cylinder in field plants for swelling, due to the larger volume 

 of the parenchyma rays; and the smaller capacity of the cortex tissues for 

 swelling, due to the larger rubber-content. The greater volume of cork 



and of the cortical intercellular spaces in irrigated plants must also be 

 considered. As these tissues are included in the tables under the term 

 "bark," it is obvious that an error is introduced which is larger for the irri- 

 gated plants. Hence the ratios ought to be, for these, relatively smaller. 

 Table 44 shows the same relations for branches of larger size, in which 

 the ratios of bark to wood are smaller, but relatively more so in irrigated 

 plants. The figures are of special interest, as they include the ratio seen 



Table 43. Transverse dimensions of terminal twigs of irrigated and field plants of 

 the same initial size, before and after drying {fig. 15). 



in a plant from the Hacienda de San Isidro, near Escalon, Chihuahua, 

 where guayule is said to grow rapidly. While the rate of growth is not as 

 great as supposed, nevertheless it is sufficiently so to be reflected in the 

 structure of the stem, which is intermediate in character between Cedros 

 field and irrigated plants. In the three cases the initial wood cylinder 

 diameter (20 mm.) was the same in this way the largest available sizes 

 could be compared. The thickness of the cork is, when wet, nearly the 



