486 LECTURE XIX. 



mature, but exhibit it most conspicuously when they are about 

 three-fourths grown. An exception to this general rule is 

 afforded by the tendrils of Echinocystis lobata which retain their 

 irritability for a short time after they have ceased to circum- 

 nutate, that is in fact, to grow. Darwin has also found that 

 the irritability of tendrils is localised. In the case of most 

 tendrils, the lower or basal part is either not at all sensitive, 

 or it is sensitive only to prolonged contact. Most tendrils have 

 their tips slightly but permanently curved or hooked, and their 

 irritability is localised in the concavity of this curvature : those 

 of Cobcea scandens and of Cissus discolor are irritable on all 

 sides : the inferior and lateral surfaces of the tendrils of 

 Mutisia are sensitive, but not the superior surface. 



In some cases the effect of the stimulus is confined to the 

 point of contact, whereas in others its effect is manifested by 

 portions of the tendril at some little distance from the point 

 of contact. As an instance of the latter de Vries' observation 

 may be mentioned. He found, namely, on stimulating a 

 tendril of Cucurbita and immoveably fixing the stimulated 

 region, that the tendril curved sharply for a distance of 5 mm. 

 and 4 mm. respectively above and below the point of stimu- 

 lation. In cases of this kind there is evidently a transmission 

 of the stimulus. 



The consideration of this point naturally leads us to briefly 

 describe the general mode of action of tendrils. As a rule, 

 tendrils are in active circumnutation at the time when their 

 irritability is at its height, and the internodes which bear them 

 are, in certain cases, circumnutating as well. The tendrils 

 thus range over a large area, and consequently there is con- 

 siderable probability that they will come into contact with 

 some body around which they can twine. 



With regard to the circumnutation of tendrils and of the internodes 

 bearing them, Darwin gives the following information : in Cissus, Cobasa, 

 and most Passiflorae, the tendrils alone circumnutate : in other cases, as 

 Lathyrus Aphaca, only the internodes circumnutate, carrying with them 

 the motionless tendrils : lastly, neither internodes nor tendrils circum- 

 nutate, as in Lathyrus grandiflora and Ampelopsis. In most Bignonias, 

 Ecremocarpus, Mutisia, and the Fumariaceae, the internodes, petioles, 

 and tendrils, all move harmoniously together. 



