37 7 
Curvature of Tendrils. 
other hand, this exudation does not occur in Cobaea scandens 
and other plants which exhibit a high degree of contact- 
irritability. When a stimulus is given to the perceptive 
zone, the region stimulated curves ; if the stimulus is con- 
tinuous the curvature strengthens, but the stimulus is not 
conducted farther than 2 or 3 cm. in either direction, 
according to Pfefifer and to my own observations. I have 
recently found that when tips of actively nutating tendrils of 
Passiflora incarnata were quickly cut away by means of a 
razor-stroke given from above, without otherwise touching the 
organ, a curvature of 90° was induced at a distance of 1 cm. 
from the cut surface, an effect which so far has not been 
obtained in any other plant. 
The forms embraced under the general term of tendrils 
embrace organs of widely different anatomy and morpho- 
logical derivation. This and the fact that the degree of 
vegetative activity, as well as the condition of, and manner in 
which, these organs accomplish the fixation of the plant-body 
to a support, will show the futility of the attempt to assign 
the mechanism of curvature to one type which shall apply to 
all those organs. The conclusions reached in this paper are 
therefore restricted to the actual forms of the Cucurbitaceae 
and Passifloreae examined. In any case, however, it is to be 
noted that it cannot be assumed a priori that the curvature 
of tendrils is similar to that of stems. An essential difference 
between the curvature of stems and of tendrils is based upon 
the primal fact that the curvature of a stem implies minor 
physiological changes in the curved organ, while this process 
in a tendril entails a complicated series of complete alterations 
in functions of the first magnitude. Furthermore, a sharp 
distinction is to be. made between the processes of the 
formation of curvatures and that of loose coils. The first is a 
reaction to the stimuli to which a tendril will respond, while 
the second is consequent upon the maturity of the organ, 
a fact pointed out by Penhallow (XVII, p. 72). It has been 
repeatedly demonstrated that contact-stimuli are not conducted 
farther than 2 or 3 cm. at the most, and the formation of loose 
C c 2 
