N e w c m b e , Thigmotropism of terrestrial roots. 73 



2 loops certainly extended to tlie proximal liniit of tlie elongat- 

 ing Zone, since about 3 mm of the tip hacl to be left free to 

 curve below tlie lower loop. The seedlings were attended for 

 29 liours. They did not grow well, thougli tlie temperature of 

 tlie water was 24 ^ C. Readjustments of the loops were needed 

 biit 3 times in tlie period. Three roots became positive, 2 be- 

 came negative, and tlie otlier 2 gave probably distortion ciirves. 

 Tlie possibility is, of conrse, tliat all tlie cnrves were due to 

 tlie known tendency of tlie roots of tliis plant to distort in 

 water. Five separate tests of the roots of Raphanus sativus 

 were made by hanging over each root 2 loops of collodion 

 membrane: In the first experiment, with 2 roots used, one be- 

 came positive and one remained straight; in the second, 2 be- 

 came positive and 2 remained straight; in the third, all 4 roots 

 became positive; in the fourth, 1 became positive and 2 remain- 

 ed straight; in the last, 2 became positive, 3 negative, and 5 

 remained straight. No reason can be assigned to explaiii the 

 negative roots in the last experiment. The roots of Raphanus 

 sativus do not distort in water. The evidence, such as it is, 

 favors the notion tliat the roots of tliis plant are thigmotropic, 

 nd that a considerable area of surface niust be stimnlated to 

 bring a response; for when the single loops were hung against 

 the roots but 3 in 30 responded, whereas, when the 2 loops 

 were used on a root, 10 in 23 became positive. 



Finally to the pendulums already described were attached 

 looped bristles and silk tliread, and several of tliese loops were 

 hung over each root. It was thought that by this device a 

 larger area of root surface inight be put under pressure than 

 was done with the loops of collodion membrane. AVhen, as was 

 done in some cases, 12 of these pendulums were used for each 

 root, they did not all pull in precisely the same direction, but 

 were spread out so that their extremes of direction were about 

 25 " apart. This Separation of the pendulums was made neces- 

 sary in order to avoid interference of the loops with one an- 

 other. The only species tested by this means was Raphanus 

 sativus. Sixteen roots had each 4 bristles hung over the elon- 

 gating Zone, the combined pull not being sufficient noticeably 

 to deflect the root from the vertical direction. The bristles were 

 merely beut double, without breaking, and the wire of the 

 pendulum was twisted about the ends of the bristle so that the 

 looped bristle extended horizontally from 2 to 3 cm from the 

 wire of the pendulum. Ten roots among the 16 became posi- 

 tive, and the other 6 remained straight. The 16 seedlings were 

 divided into 4 experiments. The duration of the tests varied 

 from 4 to 21 hom^s. Some roots beut 70 minutes after the be- 

 ginning and some not for 12 hours. The temperatm^e of the 

 water was 24 ^ C. The bristles were moved downward on the 

 elongating zone usually every 2 or 3 houi's. 



Eight roots of Raphanus sativus under similar conditions 

 to the foregoing had each 12 loops of fine silk thread hung 



