446 Newcombe . — The Sensory Zone of Roots . 
too small to serve as a demonstration, and it is probable that 
the caps did not cover the whole of the elongating zone. In 
the present paper not only was the whole of the elongating 
zone covered, but several millimetres beyond. It may with 
safety be said that the root of Zea Mays (popcorn) is sensitive 
at a place io mm. back of the limit of the elongating zone ; 
that of Fagopyrum esculentum , 9 mm., or more, from the 
elongating zone ; that of Helianthus annuus , 8 mm., or more, 
from the elongating zone ; and those of Brassica alba and 
Raphanus sativus , certainly 10 mm. and probably 15 mm. to 
ao mm. from the elongating zone. 
As to the velocity of the water-stream causing these curves, 
it should be noted that it was not so great as given in the 
tables, since there no account was taken of the resistance offered 
by the glass-tubes and the roots. Elsewhere 1 it has been 
shown that velocities of current between 100 cm. and 500 cm. 
per minute bring good rheotropic responses in the uncovered 
roots of eighteen species of plants tested. It may well be 
believed that in spite of the obstruction offered by the glass- 
tubes and the roots, the velocities of flow fell within the 
limits for good responses. 
The results here given should not be taken as showing the 
limit of the sensory tissue. With better means, not only 
might the limit of sensitiveness be extended, but a greater 
percentage of curvatures might be obtained. In my work on 
rheotropism 2 it was found that many individuals of many 
species did not show a response for twelve to twenty hours 
after the application of the stimulus of the water-stream. 
It is however, when glass-tubes are used, not advisable to 
continue an experiment more than about twelve hours. 
A longer period brings distortion to several of the roots. 
These distortions are apparently due to want of exchange of 
water in the tubes surrounding the roots, the excreta from 
the roots accumulating in the imprisoned water. It is 
probably not due to a lack of oxygen ; for the roots still 
1 Newcombe, Bot. Gazette, xxxiii, 1902, p. 177* 
3 Ibid. 
