1902] THE RHEOTROPISM OF ROOTS 183 
_ Sensitive have their roots immersed in a current of water of suit- 
able velocity, the roots bend with a greater or less angle against 
the stream, thus opposing the mechanical effect of the streaming 
water and the response to gravitation. The angle of curvature 
in this rheotropic response, as in other phenomena of irritability, 
is in the growing zone, and there is a definite latent period and 
alter-effect. The angle of curvature in sensitive plants ranges 
: from a few to go°, thus bringing the tip of the root into the hori- 
_ ‘natal position. Rheotropism of roots is not a general phenome- 
ton, and great variation in the degree of response is displayed 
by different species, 
B. RESULTS WITH SEEDLINGS 
1. DISTRIBUTION AMONG PHANEROGAMS. 
1. Plants unsuited to experiment. 
7 Everyone who has made any considerable number of water 
Sige with seedlings has learned that various species or varie- 
a duce in water only badly contorted primary roots. 
’ ps such plants cannot be used to any great extent in 
| Sudy of rheotropism. Zea mays, in the four varieties of 
and Sweet corn employed, showed itself to be practically 
aE - experiment with primary roots. The variety of : 
~ , _-¢ Was a. Western dent; and of sweet corn, the White 
’ Minnesota, and Old colony. One hundred and twenty- 
uals of these four varieties were grown in water till 
— from a few millimeters to 5° in length, and — 
Toots, | Produced in still water 30 per cent. of straight 
" -™ Many cases the tip of the primary root was lifted 
mate ; in others, the root pursued a zigzag course — 
k ‘ pe in others the direction was an irregular spiral. — 
tee. i. — that Jénssen and Berg for the main part 
‘tO some ee 3 this erratic Zea mays, and Juel employed a 
. lead All three authors, however, call attention to 
Aumber of contorted roots formed. My own results 
small number of straight-growing roots 
| he telativel y 
