INSTANCES OF SPECIFIC TROPIC IRRITABILITY 227 



the cells exert upon each other in virtue of their weight, tendency to 

 growth, and mode of union, but in rejecting these conclusions Noll and Jost l 

 have forgotten that Czapek was merely attempting to give a comprehensible 

 means of arriving at the required regulation. 



There can be little doubt that, as in all vital phenomena, not only 

 the motory but also the sensory processes are connected with chemical 

 changes, and Czapek has, in fact, found that such changes do occur as 

 the result of tropic stimulation. Since they begin before any reaction is 

 shown they appear to be more or less directly related to the process of sensa- 

 tion. The change is evidenced by an increased reducing action upon an 

 alkaline solution of silver in the geotropically stimulated root-apex, as well 

 as by the reduced oxidatory action upon readily oxidizable reagents such 

 as guiacum. The change is propagated from the sensitive apex to the 

 elongating zones behind, reaches its maximum about the time curvature 

 begins, and then dies slowly away again, so that by -the completion of 

 the curvature the tissues are once more normal. Czapek's later researches 

 appear to show that the silver reduction is due to homogentisinic acid, 

 and that the latter is produced by the oxidation of tyrosin. Normally 

 the acid appears to undergo further oxidation, which is, however, suspended 

 in the presence of antioxydase ferments 2 . These are produced on 

 geotropic stimulation and are responsible for the accumulation of the 

 reducing substances in the cell. The latter might, however, equally well 

 be the result of an increased productive activity only indirectly connected 

 with the tropic stimulation. 



Similar results have been obtained with hydrotropically stimulated 

 roots and also with the heliotropically stimulated seedling stems of a few 

 plants. The increased reducing action is not, however, produced by 

 diffuse illumination, or in roots from which the sensitive apex (i^ mm.) has 

 been removed, so that the result is due to tropic stimulation. It does not, 

 of course, follow that all plants will react in the same way, and to all 

 forms of stimulation ; but if these changes prove to be a constant accom- 

 paniment of tropic stimulation they may serve as indications of the 

 latter when the power of movement is absent, or when the stimulation 

 is not intense enough to excite it. Czapek found that the reducing 

 substances appeared in equal quantity on both convex and concave sides of 

 a curving root, so that the unequal distribution of growth appears to have 

 a different origin. 



PHOTOTROPISM. Phototropic excitation is dependent not only upon 



1 Cf. Noll, Jahrb. f. wiss. Bot., 1900, Bd. xxxiv,p. 465 ; Jost, Biol. Centralbl., 1902, Bd. XXXII, 

 p. 165. 



2 Czapek, Jahrb. f. wiss. Bot., 1898, Bd. XXXII, p. 208 ; Ber. d. bot. Ges., 1901, p. 122 ; 1902, 

 pp. 454, 464; 1903, pp. 229, 243. Cf. also Noll, Jahrb. f. wiss. Bot., 1900, Bd. xxxiv, p. 485. 



3 On antiferments cf. Czapek, Ber. d. bot. Ges., 1903, p. 229. 



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