66 Newcombe, Thigmotropism of terrestrial roots. 



misinterpretation of resnlts , resort was taken to experimentation 

 wlien th.e roots were inimersed in water. 



At first tlie simple niethod was eniployed of allowing roots 

 to grow down against cylindrical surfaces such as tliose of corks 

 and of crystallizing dishes sanken in tlie water. Tlie seedlings 

 were so placed that their root-tips met tlie corks or dislies but 

 a little way above tlie horizontal axial plane of tlie latter; thus 

 tlie roots would be but slightly deflected froni tlieir course as 

 they grew down past tlie object. It was thought that if the slop- 

 ing side of the root was sensitive to pressure, the root niiglit 

 apjiress itself to the stiniulating object and follow its curving 

 surface. Brassica alba and BapJianus sativus were used in tliese 

 tests. Only 18 seedlings were employed, since the metliod was 

 soon abandoned. The objection was that by the growtli of the 

 root, its tip was in iiiany cases mechanically forced out of con- 

 tact witli the cylindrical surface. Howewer, 6 of the roots kejit 

 their tips appressed to the ojiposing surfaces, and turiied back 

 beyond the vertical direction wlien the horizontal axial plane of 

 the cylinder was passed. 



The foregoing exjieriments sliowed plainly enougli that wliat 

 is desirable to give pressure against the root is not a fixed body, 

 but one that will yield and advance witli the root. To effect 

 this end, collodion sacs a centimeter in diameter and 4 to 5 cm 

 long witli very thin walls were made. To prepare them, sugar 

 was melted and rolled into the desired size. Into one side of 

 the sugar-rod, a glass-tube of a few centimeters in lengtli was 

 thrust. This object was tlien dipped into very thin collodion 

 Solution, and as soon as dry fastened by parafiin to a glass- 

 plate of suitable size. Wlien subsequently the preparation was 

 inimersed in slowly running water and left for 12 liours, the 

 sugar was completely dissolved out, leaving a collapsable cylin- 

 der of collodion , supjiorted by the glass-plate. The notion was 

 that by sinking this object in water, and allowing water to flow 

 very gradually into the open end of the glass-tube wliicli jiro- 

 jected above the level of the water in the jar, a very gentle 

 resistance could be brought to a root impinging upon the col- 

 lodion membrane. It was hoped that this niiglit bring a re- 

 sistance wliicli would be neaiiy constant, but wliicli could be dis- 

 placed by the advancing root. 



The hope was but poorly realized. It was soon found that 

 the slightest elevation of water-level in the glass-tube above that 

 of the surrounding water made the collodion cylinder too re- 

 sistand to be creased by the growing root. The resiüence of the 

 collodion membrane could be overcome by the roots, but the 

 membrane did not offen iit itself closely against the root. Nine 

 seedlings of Zra mays (everta Sturt., popcorn) and 10 of Bn- 

 plianus sntlvus were employed in this exjieriment. The former 

 gave 5 and the latter 7 weak positive curves, while several roots 

 beut negatively, following the curve of the membrane, instead 

 of pushing it aside. 



