356 BOTANICAL GAZETTE [may 
majority of roots in any one preparation were unaffected. Rheo- 
tropism, therefore, is not aerotropism. 
Lastly, we may ask what the probability is that rheotropism 
is response to pressure. On the one. hand we know that the 
water current exerts one-sided pressure on the roots, and we are — 
at a loss to perceive any other influence which the current may 
exert. Onthe other hand,a response of roots to pressure, either 
a positive or a negative response, has never been demonstrated. 
Sachs thought he had found roots positively thigmotropic when 
stimulated on the elongating zone; but the curves produced 
were traumatic, as a forthcoming paper of mine will show. 
_ Darwin’s ” supposed negative thigmotropic curves of the root tip 
have been shown to be traumatropic by Wiesner and Spalding.” 
If the rheotropic curving is due to pressure, it might be 
thought that solid particles in the water stream might cause 
_ either an earlier or a greater response. Two experiments were 
_ tried with seedlings of Helianthus annuus, in one of which wheat 
_ flour was stirred into the water stream and in the other clay was. 
used. In each case a control was set up with roots in 
_ filtered through a Berkefeld filter. While the water was revolv 
2 the solid material was stirred up every half hour. In the eC 
test the roots gavea readier response in the filtered water ac ae 
that. containing the flour; in the second case, responses app* 
about the same time and the roots attained ae | 
ae: in both basins. ae 
: There is no evidence to show that roots are “sensitive 1 
: pressure, and until that is shown it is idle to apeenlatgy as to the 
telation of een and thigmotropism. . 
E. SUMMARY. 
ee I, STATEMENT OF CHIEF RESULTS. : 
oo ‘The foregoing pages narrate the results obtained 
ment with thirty-two species of plants. These ee 
“Arbeit. aus. d. bot. Inst. Wiirz. : 437. 
7 Power of movement in plants, 1880, p. 129. 
*Das Begwegungsvermigen der Pflanzen, 1881, p. 139. 
On =) Nnemea marie of roots. Annals of acon! 8: ee 1804 
