IRRITABILITY. 479 



deal here with phenomena resembling those of heliotropism 

 and of geotropism. Here, too, we have a stimulus acting upon 

 a growing organ which responds by altering its direction of 

 growth, by becoming curved ; in this case also heterauxesis is 

 induced, the curvature being due to the more active growth of 

 the convex as compared with the concave side. The resem- 

 blance is further brought out by the observations of Molisch, 

 that, in the hydrotropic curvature, as is usually the case in 

 the heliotropic and geotropic curvatures, the region of greatest 

 curvature, in roots at least, coincides with the region of most 

 active growth. Until the time of the publication of Darwin's 

 observations, it was thought that the hydrotropic irritability, 

 like the geotropic irritability, resided in the curving cells ; but 

 Darwin came to the same conclusion, from his experiments, 

 with regard to hydrotropism as with regard to geotropism, that 

 the irritability is localised in the root-tip, and that the stimulus 

 is transmitted to the actively growing region. He found, 

 namely, that when the tips of the roots of seedlings (Phaseolus, 

 Vicia Faba, Avena, Triticum), were coated for a length of I 

 or 2 mm. from the apex with a mixture of olive-oil and 

 lamp-black which formed a water-proof covering, or when the 

 tips were cauterised with nitrate of silver, the roots performed 

 hydrotropic curvature rarely and in a slight degree only. 



Darwin's experiments were made in the following way. The seeds 

 were sown in damp sawdust contained in a sieve inclined at an angle of 

 40 degrees with the horizon. After germination the radicles protruded 

 through the holes of the sieve, and were exposed to the hydrotropic 

 influence of the inclined surface of moist sawdust : the projecting radicles 

 could easily be coated with grease or cauterised. 



The following may serve as an example of his numerical results : 

 Phaseolus multiflorus. 



a. Of 29 untouched radicles, 24 curved hydrotropically so as to come 



into contact with the sieve. 



b. 8 radicles had their tips greased for a length of 2 m.m., and 2 radicles 



for a length of i^ m.m. 



for the first 24 hours they were all either vertical or nearly vertical, 

 some had curved towards the damp surface to the extent of 10. 



at the end of 48 hours, three of the radicles had become consider- 

 ably curved towards the sieve. 



