Anatomy and Histology. 121 



EFFECT OF VARIOUS AMOUNTS OF WATER OF IRRIGATION. 



The most fundamental economic question for which an answer will be 

 sought in these pages is that relating to the production of rubber by plants 

 under irrigation. As bearing upon the answer is the relation of the above 

 tissue-responses to the amount of water supplied, as already indicated in 

 table 44. That the inference based upon the data there displayed is cor- 

 rect is indicated by the measurements taken from irrigated plants from 

 two localities where the conditions were unavoidably and markedly dif- 

 ferent, as follows: 



Cedros. Stocks planted March 1907, by Mr. C. T. Andrews. Irri- 

 gated freely till April 1908. Went dry till summer rains. 25 cm. growth 

 during 1907 and during 1908. Long drought from September 1908 to 

 May 1909, but probably irrigated somewhat during this period. Sample 

 plant collected May 10, 1909 (plate 17, fig. B). 



Caopas. Whole plants of medium size planted January 1908, and 

 abundantly irrigated till June 1908. On account of failure to start in 

 January they were trimmed back down to the stouter branches. New 

 shoots then started, these being for the chief part included above under 

 "total diameter 3 mm. or less." The Caopas presa broke out in June 

 1908, so that between that date and the time of collection (May 9, 1909) 

 no irrigation was possible (plate 46, fig B). 



Table 48. Comparison of ratios of bark to wood by weight for plants from 



Cedros and Caopas, irrigated. 



From the figures it may fairly be concluded that the amount of dis- 

 turbance in rate of growth in the tissues considered is, within certain wide 

 limits, related to the amount of available soil- water. The less the water, 

 the thicker the bark (cortex) , and vice versa. The Caopas plants certainly 

 had less water than the Cedros plants, and the ratios of bark to wood 

 stand in these at 1.56 and 1.16, respectively, for the small branches which 

 grew in both plants under irrigation. As to the reduction in radial meas- 

 urement of the chief rubber-bearing tissue, the cortex, it must be remem- 

 bered that this is compensated for by the more rapid growth of plants 

 under irrigation (up to six times) , so that the absolute amount of cortical 

 tissue in an irrigated plant will be greater than that in a field plant for the 

 same period of growth. 



The rate of growth determines the total volume. In order to obtain 

 an empirical factor for the purpose of conveying to the mind an approxi- 

 mate notion of the relative ability of field and irrigated plants to produce 

 "bark" in a given period of time, I took two average twigs of one season's 

 growth, removed the leaves, decorticated, and weighted. The figures in 

 table 49 were obtained. Here it is evident that, aside from the much 

 more rapid growth in weight in irrigated plants, the amount of rubber- 



