IRRITABILITY; POLARITY AND CORRELATION 353 



tion. How these changes in the permeability of the cells are 

 brought about still remains to be solved. That the drooping is 

 not a mere pull of gravity is shown by the fact that if the plant 

 is inverted and stimulated the leaves fold up towards the roots. 

 When the turgor is released on the basal half of the pulvinus, the 

 tissue tension on the apical half consequently forces the drooping 

 of the leaves. 



The past few years the interest in this plant has centered largely 

 on (1) the region of the stem and petioles in which the stimulus is 

 transmitted, (2) whether living cells are necessary for the trans- 

 mission, and (3) whether the stimulus is hormonic or nervelike in 

 nature. Among the chief workers in the field have been Haber- 

 landt, Bose, Ricca, Ball, and Snow. Haberlandt thought the stim- 

 ulus was carried by water waves in the elongated cells of the 

 phloem. Ricca (1916) found that the stimulus would travel 

 through the stem even when the stem was girdled for six inches or 

 if killed by heat, and he concluded that the xylem was the chief 

 conducting region. When cut in two and separated by water in 

 a glass tube, the stem below exuded a greenish substance, which 

 passed through the tube and caused a reaction in the part above. 

 Snow (1925) concluded that the stimulus traveled in the xylem in 

 the stem but transferred to the phloem at the pulvini of the leaves, 

 continuing from there in the phloem. Ball (1927) found two types 

 of conduction— one at a rather slow rate (20 cm. per minute) in 

 the xylem, and a more rapid or " explosive" type (200 cm. per 

 minute) through the pith. In this latter case, the conduction of 

 the stimuli was associated with the collapse of the turgid pith 

 cells, which as they contracted seemed to eject a substance that 

 caused the contraction of neighboring cells. These facts and 

 theories all point to a hormone traveling in the transpiration 

 stream, but objections to this conception are the facts that the im- 

 pulse travels in both directions (up and down) and faster than the 

 transpiration stream. Bose, from a series of elaborate experiments, 

 came to the conclusion that the transmission is in the phloem and 

 by some mechanism similar to that in the nerves of animals. The 

 famous Austrian physiologist, Molisch, went out to India as a 

 sort of representative from the West to see just how much Bose 

 was enlarging his results with the very sensitive auxographs he 

 has constructed, but, much to the surprise of the Occident, Mo- 

 lisch became a convert and has supported the main contentions 



