80 N e w c o m b e , Thigmotropism of t errestrial roots. 



were open and some were closed at tlie lower end. The iulics 

 collapse better aboiit tlie roots wlien tlie lower, pointed, tip is 

 snipped off. Wlien such open tubes were used, they always 

 projected several millimeters below the tip of the root. The coUo- 

 dion tubes never covered less than the distal 25 mm of the root, and 

 most of them covered over 30 mm, while some roots were wholly 

 shielded from the water-stream. The veloeity of the current 

 where it met the roots varied from about 450 cm per minute 

 in the outer part of the basins in the more rapid revolutions 

 to 250 cm per minute nearer the middle in the less rapid 

 revolutions. 



In three tests made with the roots of Zea mays (popcorn), 

 each test showed a majority of the roots covered with the col- 

 lodion tubes curved against the stream. Fifteen roots curved 

 positively in a total of 24 with the collodion tubes lying closely 

 against them, 2 roots made negative bends, and 7 remained 

 straight. Four of the positively curving roots had 25 to 30 mm 

 of the apex covered, and the other 11 roots had 30 to 35 mm 

 covered. Roots of Baphanus saüviis to the number of 18 were 

 covered by the collodion tubes , and 14 of them responded with 

 positive curves, while one became negative , and 3 grew straight. 

 Seven of the positive curves were in roots wholly covered 

 from the water, and the other seven had 27 to 35 mm of their 

 apical portion covered. Twenty-one roots of Lupinus albus witli 

 collodion tubes covering them in Streaming water showed 15 pos- 

 itive curves, 4 negative, and 2 neutral roots. Three of the pos- 

 itive roots were wholly covered by the collodion, and six others 

 were covered to the extent of 30 mm or over. 



To my mind there is no doubt that these positive curves 

 were thigmotropic , nor that rheotropism is thigmotropism. Un- 

 covered roots as controls, in the same basins of water as those 

 covered by collodion, usually began to curve a little before 

 those with the envelopes — the controls usually in 2 to 6 hours, 

 while those in the tubes were generally one to 2 hours later. 

 The curves in the controls and those covered looked alike, ex- 

 cept that those of the controls were usually the stronger. How- 

 ever, the positive curves in the roots covered by the tubes 

 were nearly all over 20 " and many of them over 45 ". The cur- 

 ves were as great in tlio roots whoso collodion stockings were 

 closed at the bottom and covered the whole extent of the root 

 as in those cases in which the stockings were open at the bot- 

 tom, and covered only 25 or 30 mm of the apex. Conclusive 

 evidence that it is not a filtration stream through the collodion 

 tubes which causes the curves is furnishod l)y the behavior of 

 roots not straight enough to aUow their enveloping tubes to lie 

 closely on their stream- ward side, the pressure of the water not 

 being great enough to force the membrane into contact with 

 the root. Twenty such preparations distributed among the 3 



