122 



Guayule. 



bearing tissue formed by a single twig is at least 5.5 times that produced 

 on a field twig of similar age. 



By introducing this factor, and that of rapid growth, into the calcula- 

 tion, it may be seen that the resulting total volume of the rubber-bearing 



tissue preponderates in the most 

 Table 49. rapidly grown plants, and from the 



data set forth there emerges the 

 conclusion that it is possible to reg- 

 ulate irrigation, and thereby to pre- 

 determine, within the usual limits 

 approximately shown in the preced- 

 ing pages, the relative total amount 

 of cortex and wood. I do not forget 

 that difficulties of another sort, 

 related to the manufacture of crude rubber, may be introduced, but with 

 these we are not at present concerned. The remaining part of the ques- 

 tion, as to the amount of rubber the tissues of irrigated plants are capable 

 of producing, is in part answered beyond, in a succeeding chapter. 



EFFECT OF DROUGHT FOLLOWING IRRIGATION. 



From the ecological point of view, it seems reasonable to argue that 

 the greater production of parenchymatous tissues is in the direction of suc- 

 culency, and is an adaptive response to the arid conditions under which 

 the plant lives. The largest growth of these tissues is found in the paren- 

 chyma rays as well as in the cortex, and there can be little doubt that the 

 parenchyma of the pith, parenchyma rays, and cortex function to some 

 degree of efficiency as water-storage reservoirs. It is, however, clear from 

 the measurements which have been presented in the foregoing tables that 

 the way in which this succulency works is not by capacity for a large 

 amount of water irrigated plants are superior in this respect but, it 

 must be argued, in holding it more tenaciously. The efficiency in this 

 direction is, however, not very great, if we measure it crudely, as when we 

 observe the rate of wilting when the plant is removed from the ground, 

 and it is not in any sense to be compared with the resistance of desert suc- 

 culents in this regard. What the rubber itself may contribute to this mod- 

 erate efficiency can be answered only in speculative fashion. The death 

 of large numbers of plants scattered over large areas after severe drought 

 does not warrant extravagant notions, at any rate. 



EFFECT OF IRRIGATION ON THE PHYSICAL CHARACTERS OF 



THE WOOD. 



The wood of irrigated plants is noticeably harder and more rigid than 

 that of field plants (Lloyd, 1 9086) . This is apparent upon cutting or upon 

 twisting or bending. For the purpose of measuring the differences in flex- 

 ural rigidity, two slender wood cylinders of equal (2 mm.) diameter were 

 obtained by freeing them from the cortical tissues, and were then sub- 

 jected to bending before and after drying. It was found that, when still 

 wet, the wood of the irrigated plant is three times more rigid than that of a 

 field plant (the exact ratio was 11 to 3.5) and when dry the ratio is 2 to 1. 



