Anatomy and Histology. 



117 



control material softened in a moist chamber. But the small error, less 

 than i per cent, is of significance only when such data are used in large 

 calculations. 



From the present point of view a volumetric method is of more value 

 than weighing, since the ratios derived by the latter are disturbed by vari- 

 ation in the specific gravity ; but as a comparison 

 of the ratios derived by both methods is of use in 

 practice, they have both been introduced. For 

 similar reasons it is im- 

 portant to know the 

 ratios derived from dry 

 material, and for this 

 purpose the method of 

 displacement of alcohol 

 has been used. 



RELATIVE VOLUMES 

 OF CORTEX AND WOOD. 



The material from 

 which the data tabulated 

 in table 42 were obtained 

 was collected at Cedros. 

 The irrigated material 

 was taken in September 1908 

 from stems (fig. 16) of two sea- 

 sons' growth. The difference in 

 thickness of wet and dry cortex 

 is very slight, and is not given. 



It is seen that the volume 

 ratio of bark to wood (when dry) 

 in the irrigated plant is near to 

 unity in the smaller twigs to 0.27 



FIG. i 7 .-Relative thickness of cortex in stems of irri- in the l&T Z eT ' U P tO * '^meter 

 gated and field plants, the wood cylinders being of 13.5 mm., beyond which no 

 of equal diameter when dry. i , >, T ^ u 



material was available. In field 



plants the ratio for the smaller twigs approaches 2.5, being reduced to 

 1.7 for stems 13 mm. in diameter, and still further, namely, to unity, for 

 stems exceeding this diameter (20 mm. and more). From the economic 

 point of view this material reduction of cortical tissues in irrigated plants 

 is an important consideration, since it is these tissues which contain the 

 rubber. 



The ratios for the wet tissues indicate the large water-holding capac- 

 ity of the irrigated cortex, especially as compared with the field material. 

 These differences in volume are quite obvious in the radial measurements 

 of the wood and bark. In tables 43 and 44 some accurately made 

 measurements are given. The ratios are illustrated in figs. 15, 16. For 

 the better direct comparison of field and irrigated plants dry twigs of the 

 same initial total diameter were chosen and were measured both dry and 

 after being soaked in water. The initial size is shown in the diagrams 



