1 66 The Sense-Organs of Plants. [Sess. 



for the perception of very various stimuli, such as differential 

 moisture distribution, gravity, variations in light intensity and 

 direction, and contact. 



Hydrotropism and Eheotropism. — The direction of growth 

 of roots is influenced not only by the distribution but also by 

 the movements of water in the substratum. In 1872 Sachs 

 ('Arb. d. b. Inst, in Wurz.,' 1872, i. 209) discovered that roots 

 grow towards the moister side of the medium. His experi- 

 ment in demonstration of this phenomenon — Hydrotropism — 

 is the well-known one given in the text-book (Sachs, ' Lect. 

 on the Physiology of Plants,' 1887, p. 715). To Darwin 

 ('Movements of Plants,' 1880, p. 181) we owe the first 

 experimental demonstration of the localisation of hydrotropic 

 irritability. He found that roots whose tips for a length of 

 1-2 mm. were coated with a mixture of grease and lampblack, 

 and which were thus rendered impervious to moisture, showed 

 no curvature, while uncoated roots curved towards the 

 moister side. He concluded that hydrotropic irritability was 

 localised in the root-tip, — that the latter was the percipient 

 organ. Pfeffer's experiment ('Physiology of Plants' 1906, 

 iii. 198) of papering the root- tip is confirmatory of Darwin's 

 conclusions. 



A convenient method of demonstrating hydrotropic pheno- 

 mena is illustrated in Plate IX., fig. 1. Soaked mustard-seeds 

 are planted on the roof and sides of a moist chamber — an 

 inverted longitudinally bisected flower-pot. The pot is placed 

 in a plate with a little water, covered, and kept in the dark. 

 On examining after a few days, the roots of the seedlings 

 arising from the seeds on the roof are seen to be directed 

 straight downward, while those on the inclined walls, uni- 

 laterally stimulated, have followed the line of the wall. By 

 greasing or papering the tips, the latter may be shown to be 

 the percipient organs. 



That roots are stimulated to curve by movements in the 

 water in which they grow was first pointed out by Jonsson 

 ('Ber. d. hot. Gesell.,' 1883, p. 518). When the current is 

 moving with suitable velocity the roots curve against the 

 stream ; when this optimum velocity is much exceeded, a pass- 

 ive negative curvature results. These rheotropic phenomena 

 have been further investigated by Juel (' Jahr. f. wiss. Bot.,' 



