354 BOTANICAL GAZETTE [May 
roots. Of these, twenty became positive, five negative, while 
seven did not curve. 
Fagopyrum esculentum, the plants having nearly ripe fruit, and 
treated as the foregoing species, gave twenty-four roots for 
experiment. After 38 hours, fourteen roots were positive, four 
were negative, and six were contorted. 
Raphanus sativus, a month old, furnished twenty-eight roots” 
for experiment. No curves appeared for the first 20 hours. 
After 50 hours of revolution of the basins, twenty-one roots 
had positive curves of 45 to 90°, three were contorted, and four 
were straight. : 
Again it appears here that the sensitiveness of roots to @ 
stream of water is neither lost nor gained in increasing age of 2 
the plant. In the preceding chapter it was indicated that the — ‘ 
Secondary roots are less sensitive than the primary. Thesame 
conclusion is much more strongly indicated by the experiments 4 
_ just narrated. Yet one may not argue too strongly from these — 
last experiments. Plants growing in crocks are probably not 
very vigorous, and tests have shown over and over again” that 
: lack of vigor inhibits irritable responses. To cite a single case, 
_ I may say that a crock of plants of Helianthus annuus, not in 
_ vigorous condition, was kept for 60 hours over revolving ¥ water 
> with two dozen roots immersed, and no curvatures ae 
ae D. NATURE OF THE RHEOTROPIC STIMULUS. 
The nature of the rheotropic stimulus has not aoer 
mined. In the paper read before the American Association 
the Advancement of Science 3 in 1896, I suggested oe 
theotropic stimulus was pressure. Juel (/. ¢.) has remewe® : 
‘suggestion, but no one has offered any direct evidence. 
Physiological importance of rheotropism is difficult 
ceive, and hence one suspects here a case where the 1 
developed in response to some biologically important : 
_is set in motion by some other stimulus, as an a elects 
: throws: a muscle into contraction. - 
_™ Bor. Gaz. 22: 242. ee 
