82 Guayule. 



In addition to data obtained by observation of external marks, a 

 number of measurements of field plants were made by the usual labora- 

 tory method of marking the stem with India ink. The results of these 

 observations are here given: 



Station 2, quadrat 3... 6 twigs marked at the tip with a drop of ink, Jan. 6, 



1908. Growth commenced Apr. 28. Last observation Apr. 3, 1909. Measure- 

 ments as follows, in mm.: 60, 75, 70, 75, 65, 50. Average amount of growth for 

 season, 66 mm. 



Station i. 5 plants marked Jan. 3, 1908. Last observation made Apr. 3, 



1909. The mark had been destroyed on 2 plants. The total amounts of growth 

 for the 3 remaining were 30, 18, and 35 mm., making an average for the 3 of 28 mm. 

 All growth was subsequent to May 29. 



Station 3. Dec. 31, 1907. 3 marked plants showed an average growth of 

 i to 2 cm. A seedling slightly pruned showed 2 cm. new growth by July 15. The 

 rate of growth in all plants at this station was small in 1907. 



Station 6, a low gravelly ridge in the playa, Burrita. 4 plants marked Oct. n, 

 1907. On Jan. u, 1908, 2 plants showed i mm. and 2 plants 2 mm. growth each. 

 The total amount of growth till Aug. 21, 1908, was 13, 20, 20, and 10 mm., or an 

 average amount of 18 mm. This is a locality of conspicuously slow growth. 



The average amounts of growth observed in marked plants for the 

 season of 1907 were, therefore, 66, 28, 20, and 18 mm., making a grand 

 average of growth of 31 mm. The average is lower than the one above 

 deduced from measurements of field plants, but as three of the stations 

 suffered severely from drought in 1907 the rate of growth was probably 

 rather low. Our data on the whole indicate that the rate of growth of 

 guayule in the field lies somewhere between 30 and 40 mm. annually. 

 This general conclusion can scarcely be said to be too optimistic. It will 

 no doubt be questioned by those who entertain different ideas of the 

 rate of growth of this plant. The belief is current in many quarters in 

 Mexico that growth is much more rapid, it being a common saying that 

 after guayule has been cut the crop is reestablished in five years. Such 

 surprising statements were made to me regarding one locality in particu- 

 lar that I made special effort to obtain evidence. Although an attempt 

 to visit the place, some leagues to the west of Escalon in Chihuahua, was 

 frustrated, I succeeded, through the courtesy of some friends, in getting 

 a number of plants, which, though of somewhat more rapid growth than 

 usual, are not remarkable in a special degree. The plants were clean- 

 limbed and smooth-barked, the effect of this more rapid growth. They 

 bear evidence of a heavier rainfall as compared with plants from Zacate- 

 cas, but this appearance is due in part to the fact that they are of two 

 different types; in one the foliage shoot is abruptly terminated at the 

 base of the peduncle ; in the other the shoots taper out into the peduncle 

 after the fashion in mariola. The branches in the latter are thin, die back 

 readily and often for a good distance, and in these plants have some of 

 the characteristics seen in the stems of irrigated plants. I give measure- 

 ments of the few plants, which came to me for study, in detail (table 36). 



The average amount of growth of each plant for the years indicated 

 is: plant i, 30 mm.; plant 2, 41 mm.; plant 3, 37 mm.; and for all the 

 twigs on 3 well-developed plants of the first -mentioned type, viz, with 

 abruptly ending foliage-shoots, it is 37 mm. The data are instructive in 

 that they point to a " fat " year preceding two " lean " years, namely, 1907 

 and 1908. The rate of growth, however, compares very closely with 

 that derived from material from other localities. 



