N e w c m b e , Thigmotropism of terrestrial roots. 67 



Another methocl cmployed to secure a niovable resistance 

 against the advancing root-tip was that in which a rider was 

 attached to eacJi root. From a plate of coUodion al)oiit, 5 mm 

 in thickness little pieces 3 mm Square were ciit, and were fast- 

 ened togetlier at one edge by parafiin, so as to include an 

 angle of 45 °. Through tlie centre of one of the pieces a hole 

 a little larger thau the root was made. The rider was rendered 

 slightly buoyant by attaching the proper amount of paraftin. 

 When the roots were ready and immersed in water, a rider was 

 attached to each by passing the root through the hole in the 

 Upper plate. The lower plate, inclined at45 0, then pressed up- 

 ward against the sloping side of the root-tip. 



The last named device was tried on the roots of Zea mays 

 (popcorn) and Raplianus sat'wus. Sixteen seedlings were used 

 and there was some slight evidence of positive cui'ves. An ob- 

 jection to this method was found in the tendency of the rider 

 to shift its Position when the root curved. 



The last and the most satisfactory device employed to give 

 pressure against the sloping side of the root-tip when immersed 

 in water consisted in the use of tongues of thin paper, thin 

 collodion , and rubber tissue. Pieces of each of these membranes 

 about 5 mm by 15 mm in size were fastened by the narrower 

 side each to an individual movable Standard, so that the thin 

 tongue projecting at its supported end horizontally in the water, 

 curved downward in an arc toward its free end. The outer 

 curved surface was brought against the root-tips. Thirty-three 

 seedlings of Raphanus sativu.s were used in this test. The ex- 

 periments lasted from 10 to 15 hours with a water-temperature 

 of about 20 ^ C. Adjustments of roots and opposing objects were 

 made at intervals of 30 to 60 minutes. Four roots made posi- 

 tive bends, 2 negative, and the other 27 grew straight. 



No one would be so rash as to claim that the foregoing 

 experiments demonstrate the thigmotropic sensitiveness of the 

 root-tip, but one must admit that the evidence speaks for a 

 feeble capability in this direction. Everyone who has experi- 

 mented with roots growing in water knows that even in such 

 well behaved species as Baphanus sativum the roots will occas- 

 sionally show unexplained bends. But a review of the fore- 

 ß-oinff results discloses the fact that the few bends that were 

 made in each test were more in the i)ositive tlian in the nega- 

 tive direction. If the roots here used do possess a feeble thig- 

 motropic sensitiveness, it must be remembered that in the meth- 

 ods höre om])loyed there are various circu.mstances unfavorable 

 to the ex])ression of such sensitiveness. The root to curve must 

 overcome its own tendency to grow straight (rectipetality), as 

 well as to overcome the effect of the Stimulus of gravitation. 

 Seeing that the ability of the root to respond to one-sided jires- 

 sure is at best feeble, it may be that an effective Stimulus must 

 operate over a relatively large extent of sm-face, or must operate 



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