190 Guayule. 



Dr. Spence adds that sugars are to be found in quantities in certain 

 barks, and that the physiological importance of these can not be doubted. 



The answer would seem to be that whatever occurs in Ficus elastica 

 can only be of suggestive value with regard to the guayule. And the 

 behavior of sugar described means that the unused residue of the sugar 

 has been cut out by periderm, just as the unused portion of any other sub- 

 stance may be. But this can not mean that everything which appears in 

 the bark must have been of use to the plant. The statement made by me 

 that the amount of rubber varies from time to time in the year does not 

 mean that the absolute amount in a particular individual is now reduced 

 and now increased. It means that the amount of rubber relative to the 

 weight of the plant is greater at one time than another, and I myself have 

 shown this to be the case. The gradual accumulation in the tissues, unac- 

 companied by any reduction, of rubber which might serve a storage func- 

 tion, this accumulation following growth, seems to completely contradict 

 the view that rubber is a reserve food. We may very well say that during 

 growth energy is diverted from the secretion, or, as I should prefer to say, 

 excretion, of rubber, and this would accord with the fact that the more 

 energy is expended in growth the slower the secretion takes place. 



In the statement to which Dr. Spence refers, when I speak of rubber 

 being cast off in a modified form I do not mean to say that this modifi- 

 cation is chemical, or that it takes place before the rubber is cast off, 

 but by virtue of (presumably) oxidizing processes which take place in the 

 cork-cells, which are now dead. This change, it seems to me, can have, in 

 the light of the evidence, no significance to the plant. It remains, how- 

 ever, to show experimentally that my view is correct, but it can scarcely 

 be denied that the evidence against it is tenuous. 



SUMMARY. 



The studies presented in this chapter may be summarized as follows : 



1 . In the root, rubber is first secreted in the primary canal-cells (plate 

 41, fig. 6), the activity spreading from this region as a center, but more 

 rapidly along the radius. At about the same time, or, judging from the 

 size of the granules seen, somewhat later, it appears in the innermost cells 

 of the parenchyma rays. Rubber appears in the root earlier than in the 

 stem in the same plant. 



2. Accumulation usually takes place in the oldest cells first; that is, 

 those in the outer zones. Thus, in the root the primary cortex contains, 

 before the maximum content for all the cells has been attained, more 

 rubber than the cells of the secondary cortex; and the outer cells of the 

 latter contain more than the inner. Accumulation (in irrigated plants 

 at least) is more rapid in the parenchyma-ray cells than either in the pith 

 or the cortex. 



In the primary cortex of the stem rubber may never appear, as, e.g., 

 in irrigated plants in which growth and, hence, secondary changes are so 

 rapid that the primary cortex does not have time enough for secretion. 



3. With one exception, namely, in. the hypocotyl, the accumulation 

 of rubber in the stem takes place earlier in the pith than in the paren- 

 chyma rays or cortex, and earlier in the rays than in the cortex. 



