174 Guayule. 



THE CONTENTS OF THE CANALS ; THEIR ORIGIN. 



The very small size of the primary canals in the root and hypocotyl 

 makes it very difficult to determine the nature of their contents. The 

 canals elsewhere are known to contain resin which, upon wounding, exudes 

 as tears, which fall to the ground and harden slowly as pale yellow, limpid 

 masses. The origin of this secretion is of special interest here. There is 

 no doubt that the resin is confined to the canals, and there is no evidence 

 that the resin occurs in the protoplasm of the wall-cells of the canal, 

 which have been spoken of as secretory. Treatment with alcohol or with 

 acetone leaves the cell-contents quite unchanged to all appearance, though 

 subsequent staining with alkanet discloses, when this is originally the case, 

 a substance which may be dissolved out by means of xylol or other appro- 

 priate solvent, namely, rubber. My own observations, therefore, give 

 support to the general view, advanced by Tschirch, that the resin is to 

 be accounted for by chemical activity in the outer part of the cell-walls 

 facing the meatus. It is not a direct result of protoplasmic activity, but of 

 enzymatic activity in the cell -wall itself. 1 It is worthy of remark that the 

 wall (secretory) cells of the resin-canals have the two-fold function of secret- 

 ing rubber (in common with the ground-tissue) within the protoplasm and 

 resin without. 



I have, however, attained no success in demonstrating a mucilaginous 

 or gummy lining to the meatus, such as is described by Tschirch (1906, 

 p. 1 1 19) in many plants, to which he ascribes the origin of resin formation. 

 But Tschirch himself confesses to a similar difficulty in studying, among 

 others, the Compositas. 



The distribution of starch in the cortex and its apparent connection 

 with the secretion of resin have been elsewhere noted. The presence of 

 tannin in the conjunctiva of the young stem, especially associated with the 

 chloroplasts, is to be noted, and recalls Tschirch's hypothesis of the origin 

 of resin from tannin. The number of Compositae which contain tannin 

 is small, relatively to the size of the group, judging from the list given 

 by Dekker (1906). 



THE ROLE OF RESIN. 



It has often been pointed out 2 that resins and ethereal oils stand in 

 relation to climatic conditions, especially those of the desert. The frequent 

 occurrence of resin in desert plants is a matter of general observation, but 

 its function is still a matter of speculation. Tschirch rightly lays stress 

 upon the occurrence of secretion-containing structures near the apex of 

 the young parts as of significance, and this has been pointed out for 

 the guayule. The evidence regarding the relation of resin to rubber 

 leads us nowhere, and no evidence is yet forthcoming as to the real role 

 of resin. 



1 Tschirch, A. DieChemie und Biologie der pflanzlichen Sekrete. Leipzig, 1908. 

 3 e.g., Tschirch, 1908, pp. 8-9. 



