68 N e Av c m b e , Tliigmotropism of terrestrial roots. 



for a considerable period on tlie same cells. Althougli ilie ineth- 

 ocls liere employed liave been varied, none of them has offer- 

 ed any of tlie -i possibly required conditions indicated in the 

 preceding lines. It may be objected that tlie niimber of specics 

 employed lias been too small for generalizations, and the objec- 

 tion would be valid. The anthor in his selection of species has 

 been influenced by the notion of the probability of identity of 

 rheotropism and thigmotropisni in the roots, and such species^) 

 were selected for the present study as were known to be highly 

 rheotropic. 



II. Testiug the Sensitiveiiess of the Eloiigatiug Zone. 



1, Preparations in damp-chambers at rest. 



The foregoing experiments having failed to demonstrate cer- 

 tainly a thigmotropic sensitiveness in the apex of roots, tests 

 will now be applied to the elongating zone ; and fii'st of all the 

 contact and pressure will be employed on roots at rest in damp- 

 chambers. 



The danger of a confusion of responses when objects are applied 

 to one side of the apex of the root in damp-chambers has been 

 pointed out in the preceding pages. If we assume, as some do^), 

 that sensitiveness to moisture is confined to the apex of the 

 root, then the application of objects to one side of the elongat- 

 ing zone in testing for thigmotropisni cannot iiicur the risk of 

 calling forth hydrotropism , provided tliese objects do not extend 

 their influence to the apex. 



Numerous examples of seedlings of Vicia faba, Lupinus 

 albus, Pisum sativum., Raphanns satnms, Fagopyrum esniJpufu))/ 

 and Zea mays (popcorn) liave liad Squares of glass, of iiiica, of 

 marble, and of filter-paper attached to their elongating zone, 

 and the adhering object has been ke])t on the lower part of 

 this zone by frequent adjustment during periods varying from 

 10 to 30 hours, and all without producing curves. 



On Yicia faha, Pisum sativum, and Buphanus sativus^ Ne- 

 mec's method of apjilying droplets of Avet plaster of Paris was 

 tried, using 44, 15 and 19 seedlings respectively. It was found 

 if a rounded droplet was allowed to liarden on the elongating 

 zone, a positive curve offen resulted, having the plaster in the 

 iiiiddle of the concavity. This is the same result obtained l\v 

 Nemec. If the plaster was laid on so as to form a thin cnist, 

 a curve never followed in my experiments. Whenever the plas- 

 ter was hroken across by the groAvth of the root no curves 

 appeared; tliey appeared only when the plaster had considerable 

 thickness and remained unbroken. It is trne that many roots 

 did not bend when the droplet of ])laster remained unbroken. 



1) Newcombe, The Rlieotropism of Roots. (Bot. Cxazette. XXXlll. 

 p. 362.) 



2) See Rother t, Flora 79. 212; and Czapek, Jahrb. f. wiss. Botan. 

 XXXV. p. 316.) 



