N e w c mb e , Thigmotropism of terrestrial roots. 69 



Bnt exaniiuatioii showed tliat in such cases tho root had freod 

 its surface from tlie grasp of the plaster tliougli the latter ad- 

 liered loosely; while in case of curvatnre, the plaster had a firm 

 grasp. The conclusion seems certain that the positive curves 

 obtained by this method are not thigmotropic, biit are due to 

 the prevention of elongation of cells by the adhesion of the 

 plaster. 



Objects used like those in the foregoing experiment exert 

 really no pressure against the root, and it is conceivable that 

 an actual pressure might bring a thigmotropic response. Sachs^) 

 thought he had demonstrated the sensitiveness of roots to con- 

 tact when he obtained curves by causing pins and wooden rods 

 to press on the elongating zone of roots lying horizontally. Such 

 results have been shown'^j to be traumatic. Experiments more 

 recent than those just referred to have been made witli 14 seed- 

 lings of Brassica alba, 7 seedlings of Phascolus vulgaris L. and 

 22 seedlings of Pisum sativum, by bringing horizontally extend- 

 ing rods of glass and of white pine [Piniis strohus) to press 

 gently against the elongating zones of the roots, while the seed- 

 lings were in their normally vertical position in a damp-cham- 

 ber. No curves whatever resulted. 



Thinking it possible that if a greater extent of the elongat- 

 ing zone were pressed against, the roots might show a response, 

 5 seedlings of Pisum sativum and 5 of Zea mays (popcorn) were 

 fastened horizontally in a damp-chamber, and against the lower 

 surface of the distal part of the elongating zone of each root 

 was brought a horizontally directed glass-rod, transversely to the 

 root. Nine of the 10 roots in making their geotropic curves 

 bent downward closely in contact with the rods. If these roots 

 were thigmotropic, might they not follow the rods at least a 

 little distance beyond the horizontal diameter? The root-tips 

 however did not pass beyond the vertical direction, and thig- 

 motropism was not indicated. 



2. Preparations with roots immersed in water. 



It could hardly be thought that roots which would not show 

 responses in damp-chambers , would show such reactions when 

 immersed in water. Yet the possibility exists; and should such 

 reactions be evident, their nature could be the more safely de- 

 iined as thigmotropic. Fifty-three seedlings of Brassica alba, 

 Lwpimis albus, Buphanus sativus and Zea mays (everta) while 

 growing with roots in water had glass-rods brought against the 

 lower part of the elongating zone. Every hour or two as the 

 tips had advanced, the rods were moved down, so as to keep 

 the rods against the most rapidly growing part. In some of 



1) Sachs, Arbeit, aus dem bot. Inst, zu Würzburg. I. p. 437—439. 



2) New comb e, Sachs' augeb. tliigmotr. Kurven etc. (Beih. Botan. 

 Centrblt. XII. p. 243.) 



