The Origin and Occurrence of Rubber. 191 



At the apex of the stem of field plants more rubber is found in the 

 pith than in the cortex after prolonged drought. 



In the hypocotyl (upper zones) accumulation of rubber takes place 

 more rapidly, if not earlier, in the inner parts of the parenchyma rays. 

 This appears to be due to a more primitive physiological condition of the 

 pith of the hypocotyl. 



4. With questionable exceptions, the accumulation of rubber is 

 earlier in the " secreting-cells " of the resin-canals than in the surrounding 

 tissues. The exceptions noted were (a) in the apex of a very slowly grown 

 field seedling, in the resin-canals of which no rubber was noted, and (b) 

 in the new twigs, near the apex of field plants. Rubber may be noted, 

 however, in the canal-cells, as in a very rapidly grown irrigated seedling, 

 though it occurs nowhere else. 



5. The amount of rubber in the cells of small seedlings l in the field is 

 relatively as great, or very nearly so, as in mature plants, except in those 

 seedlings (table 54) which have grown rapidly in the field, and which 

 have not had sufficient time for the accumulation of the full complement 

 of rubber. 



6. Rubber occurs unchanged in the portions of the secondary cortex 

 which have been more recently cut out by inner periderm. In the cells 

 arising directly from the outer or inner periderm rubber does not occur. 

 In the bark proper the rubber-bearing tissues alternate with nearly bar- 

 ren suber. Volume for volume, therefore, the bark contains less rubber 

 than the contingent living cortex which still remains unmodified. 



7. Rubber occurs in the pseudotylose tissue of the resin-canals in 

 quantities comparable to the amount found in adjacent cells. 



8. The accumulation of rubber in the new tissues of secondarily 

 thickened roots and stems is analogous to that in those still in the primary 

 condition. It is for some time absent from the newer parts of the paren- 

 chyma rays, and secretion occurs first in the innermost and outermost 

 cells simultaneously. The march of the secretion of rubber is, therefore, 

 from the base toward the tip of new shoots and from the pith and cortex 

 toward the cambium in older stems. 



9. In field plants, that is, in those subjected to the usual desert con- 

 ditions of their habitat, the accumulation of rubber is more rapid than in 

 irrigated plants. The maximum quantity is certainly not reached in four 

 months (June to September, inch, 1908) after growth commences, and it 

 is highly probable that six or more months must elapse. 



In a given cell, the amount of rubber in a field plant will generally be 

 greater at the end of one year than in a corresponding cell in the irrigated 

 plant in two years. Also, cells containing a given quantity of rubber will 

 be found nearer the apex of the stem of field plants than of irrigated plants. 

 It is probable, again, that the total amount of rubber that a cell in a field 

 plant is capable of secreting is greater than in an irrigated plant, though 

 this is not certain. 



1 Chemical analyses of entire small seedlings are misleading, because of (a) 

 the larger relative bulk of the leaves, and (b) the greater relative volume of tissues 

 partially filled with rubber, as in the case of seedlings taken after a period of growth, 

 but before the maximum rubber-content has been reached. 



