399 
Curvature of Tendrils. 
3. Tendrils, irritable to contact, exhibit the greatest dif- 
ference in morphological derivation, anatomy, and degree of 
irritability, and it cannot be assumed that their mechanism 
is identical. 
4. The curvature of such highly developed, initially dorsi- 
ventral organs as the tendrils of the Passifloreae is due to the 
contraction of the tissues of the concave side. 
5. The curvature of a tendril around a support as a direct 
reaction to irritation, and the coiling of a free portion of 
a tendril are entirely distinct, and to a great extent inde- 
pendent processes. The first is due to the action of tissues 
on the concave side of the organ, the second to the excessive 
growth of the convex side, and is probably preceded or 
accompanied by the relaxation of the tissues of the concave 
side consequent upon loss of irritability. 
6. The curvature of a tendril around a support does not 
accelerate the growth of the convex side. The growth- 
extension of a coiled portion of a tendril is rarely equal 
to that of a similar region in a freely nutating organ. 
7. The region of maximum growth lies between the middle 
of a tendril of Passiflora and the tip, and never at any time 
coincides with the region of greatest irritability. 
8. The plasmolysis of stimulated or unstimulated tendrils 
generally results in a shortening of the radius by the con- 
traction of the outer layers of parenchymatous cells of the 
concave side. 
9. The attachment of a tendril to a support influences only 
in a minor degree the growth and formation of coils of the 
free portion of the organ. Such influence is due primarily to 
the traction exerted upon the organ by the weight of the 
shoot, not to a conduction of the contact stimulus. 
10. The amount of strain capable of being exerted during 
the coiling of the free portion of a tendril is a specific char- 
acteristic, and generally is greatly in excess of the weight of 
the shoot to be supported and drawn upward by the organ. 
A tendril of Passiflora may exert a strain amounting to 10 grams, 
while that of Cucurbita may exert a strain of 30 grams. 
