Conduction of Excitation in the Leaf of Mimosa Spegazzinii

From an investigation into the conduction of excitation in Mimosa Pudica , carried out recently in Trinidad (17), it was concluded that normal conduction in the stem is correctly explained by Ricca’s now well-known theory of conduction, based upon his remarkable and ingenious experiments on M. Spegazzinii (16). According to Ricca, the stimulus sets free a stimulating substance, which finds its way into the vessels of the wood, and is there carried along with the water-current. It was also concluded, however, that Ricca’s theory cannot explain conduction in the leaf of M. pudica . for, in the leaf, Ricca’s conducting mechanism is regularly masked by another more rapid mechanism, which has nothing to do with the transpiration current, and works in the phloem. That the phloem is the path of conduction in the leaf of M.pudica had previously been shown by Herbert (14). There are, however, some experiments by Ricca on the leaf of M. Spegazzinii which he considers to show that in the leaf, as well as in the stem, conduction depends on the transpiration current (16, pp. 101 and 135). Since it did not seem likely that the two species should differ in so important a respect, it seemed desirable further to investigate this question in the leaf of M. Spegazzinii . For the opportunity to do so I am very much indebted to the courtesy of Prof. Penzig and Dr. Ricca at Genoa, and of Prof. Buscalioni and Dr. Catalano at Palermo, who kindly provided me with all the facilities of the botanical institutes and gardens at these two places, and offered every help towards carrying out the work.