70 N e w c m b e , Thigmotropism of terrestrial roots. 



the experiments the rods were made to piisli tlie roots 20 ^ to 

 30 " out of tlie vertical position , so that the ensuing geotropic 

 curve woiüd insiu'e good contact witli tlie glass-rods. The ex- 

 periments eontinued froni 12 to 30 hours, in a temperatnre of 

 IG " to 24 ö C. ISTo certain thigmotropic response eould be 

 demonstrated in any species. Among the 14 roots of Brassica 

 used, 3 weak, positive curves might be taken to indicate re- 

 sponse to pressure ; but this alone must be regarded as insufficient 

 Warrant for any conclusion. 



A plant of Vanilla aromatica Sw. growing in the plant- 

 house, and producing roots was taken to a dark-room and tilted 

 so that 3 healthy-looking aerial roots were immersed in a jar 

 of water. Against the elongating zone of each root, while still 

 in water, was brought a glass-rod. For 3 days the preparation 

 was eontinued, the roots growing well, and the rods continually 

 pressing against tliem, the latter being readjusted as the roots 

 elongated. There were no cui'ves. This result is to be con- 

 trasted witli that obtained by Ewart i) who induced curves in 

 the roots of this plant by bringing against them glass-rods while 

 the roots were in damp-chambers. The roots of the plant used 

 in my experiment were probably capable of curving, for the 

 plant had many roots grasping its supporting frame. The ex- 

 periment here cited is not offered as strong evidence against 

 thigmotropism, but merely as a caution against accepting as 

 thigmotropism in Ewart's experiment what may have been hy- 

 drotropism. The use of a damp-cluimber is not a guarantee 

 against such error. 



Another means employed to give one-sided pressure was 

 the preparation of collars of paraffin. These were made from 

 pieces about ^> mm square and one millimeter thick, perforated 

 in the middle, and cut out on one side. The perforation was 

 made a little sm aller than the root. When the bit of paraffin 

 had been held in the fingers for a few minutes it became some- 

 what soft and could be passed over the root-tip and fitted close- 

 ly to the elongating zone. Thus the root was grasped around 

 nearly two-thirds of its circumference. Four roots of seedlings 

 of Zea mays (everta) had these coUart attached to their elongat- 

 ing zones, but showed no curvature cluring a period of 10 hours 

 in water at a temperature of 28°. The period was not as long 

 as desirable, but there was partial compensation in the very 

 rapid growth resulting from the approximately Optimum tem- 

 perature. 



Tongues of collodion and rubber tissue, fastened to supports 

 horizontally at one end, were brought against roots of BapJianus 

 sativus , so that the free part of the tongue pressed witli its 

 curving surface like a weak spring against the elongating zone. 

 Thirteen seedlings were used, of which 4 beut slightly posi- 



1) Ewart, Oll contact-irritabilit^^ (Ann. de jard. bot. de Bnitenzorg. 

 XV. p. 234.) 



