662 LECTURE XXXVIII . 



Others, as those of Cohaa and Cissus discolor are so on all sides ; in Mutisia clematis 

 the lower side and flanks are irritable, the upper side not so. 



While in the condition of revolving nutation and irritabilit}', the tendrils attain 

 their complete length in a few days ; the revolutions cease, the irritability disappears, 

 and, according to the species in each case, further changes now take place. In some, 

 the fully developed tendrils become immovable, remain straight, decay, and 

 fall off — e. g. the Bignonias, Vitis, Ampelopsis. More frequently it happens that 

 the tendril on the cessation of growth in length becomes slowly rolled up with the 

 lower side concave, commencing at the tip and proceeding to the base, so that 

 it at length forms a helix {Cardiospermum, Mutisia), commonly a spiral narrowed 

 upwards into a cone (like a cork-screw), in which condition it then dries up (Cucur- 

 bitacese, Passiflorae, &c,). 



These processes, however, are to be regarded as abnormal, in as far as the 

 tendrils have in these cases failed to attain their object, which consists in their 

 coming into contact with a support by means of their revolving nutation during the 

 irritable stage and while they are still growing ; if this happens on the irritable side 

 curvature results at the point of contact, and this extends also to the neighbouring 

 untouched parts of the tendril, so that after a short time — a few minutes or hours 

 according to circumstances in each case, a coil is produced. This coil, however, 

 is by no means closely applied to the support if it is very thin, but touches it 

 only at one point of the concavity. The continued contact at this point, however, 

 increases the irritable action more and more, and the coil becomes closer, until 

 it has adapted itself to the support along a complete turn. By this means of 

 course the contact-irritation is simply increased still more, and the free end of 

 the tendril becomes curved still more and goes on closing gradually in new coils 

 around the support, until even the extreme tip applies itself closely to it. The nearer 

 the place first touched lies to the base of the tendril, the more numerous the coils 

 around the support and the firmer the holdfast ; nevertheless even a few coils around 

 the support with the end of the tendril suffice to fix this latter very firmly. 



Those parts of the tendril which are between its base and the point where it is 

 fixed are obviously not able to coil themselves around the support, although the 

 stimulus which causes the curvature is propagated to this region ; the effect of the 

 stimulus is simply that the portion of the tendril lying between the fixed point and the 

 rigid base becomes coiled in the form of a cork-screw, often with very numerous 

 coils. This coiling is essentially the same as that which occurs in most of the 

 tendrils which have matured without meeting with a support ; but it differs in two 

 respects from that spontaneous coiling. In the first place it always occurs, simply 

 in consequence of the stimulus in all the tendrils which have become attached to 

 a support, and not only in some ; further, in that it takes place after a short time 

 (several hours or a day after the seizure of the support), during a period .when the 

 tendril is still perfectly irritable, and rapidly growing in length, whereas the spon- 

 taneous coiling up takes place when growth and irritability have ceased. More- 

 over, the spiral coiling consequent on the stimulus of contact follows much more 

 rapidly than the spontaneous one. Both are easily seen on the same shoot, where 

 it is possible to find older tendrils which are not fixed and still straight, and younger 

 ones fixed and already coiled up. This coiling up of tendrils fixed to su})ports 



