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



over tlie elongating zone, eacli loop attaclied to an inclependent 

 pendulum. These 12 loops pulled on tlie root over an extent of 

 not more tlian 10 mm. It is certain tlierefore tliat each milli- 

 meter of the elongating zone feit a sliglit pressure. The man- 

 agement of so many loops for one root is somewhat difficult, 

 and necessarily the nnmber of roots operated witli could not be 

 very large. Six roots becanie positive and 2 negative in periods 

 varying from one and one quarter to 20 hours. 



The preceding experiments in which, while the roots were 

 in water, pressure was brought to bear on one side of the elon- 

 gating zone have not given results pronounced enough to justify 

 one in claiming a thigmotropic sensitiveness , though certainly 

 Baplianus sativus has given strong indications in thät direction. 

 In all of these experiments, however, the geotropism of the root 

 has opposed any thigmotropic tendency which may have been 

 present. The experiments have perhaps been sufficient to sliow 

 the absence, or extreme feebleness of thigmotropism in terrestrial 

 roots in usual conditions. But is there a thigmotropic sensitiv- 

 eness strong enough to express itself when the action of grav- 

 itation is neutralized? The following section will deal with this 

 question. 



3. Preparations in damp-chambers on klinostats. 



In experimenting for thigmotropism of roots in a damp- 

 chamber, the danger from hydrotropism has already been point- 

 ed out. Yet if one wishes to conduct tests in which gravitation 

 shall be neutralized, it seems practically impossible to operate 

 with the roots immersed in water, because of the accompanying 

 irregulär movements of the water. And it will be remembered 

 that in the preceding experiments in damp-chambers no liydro- 

 tropic curves foUowed the contact of objects with the elongating 

 zone. The damp-chambers employed in the following experiments 

 were covered glass-dishes 15 cm in diameter and 4 cm deep, 

 lined throughout with filterpaper, and having aflooring of cork 

 one centimeter thickness. The seedlings used were fastened in 

 perforated corks by packing with wet cotton, and the corks 

 were then pinned to the floor of the damp-chamber so that the 

 long axis of each seedling was parallel with the floor of the 

 Chamber. The Chambers were placed on the klinostats so that 

 the long axes of the seedlings moved always in a vertical plane. 

 In making the preparations ready for the klinostats, care was 

 taken to avoid subjecting the seedlings to Stimulation for geo- 

 tropic curvature. The Chamber was always throughly moist 

 when the seedlings were pinned in, and was well sprayed with 

 water before it was finally closed. 



In the first series of experiments with gravitation neutral- 

 ized, bits of glass 2 to 3 mm Square and 0,1 mm thick were 

 laid on the elongating zone of roots, 2 mm back from the apex. 

 The notion was that these objects would exert a pressure through 



