160 TENDKIL-BEABEnS. Chap. IV. 



thin twigs, become deeply sinuous or zigzag. More- 

 OTer the whole end of the petiole or tendril, if it seizes 

 nothing, bends after a time abruptly downwards and 

 inwards, showing that its outer surface has gone on 

 growing after the inner surface has ceased to grow. 

 That growth is the chief cause of the spiral contrac 

 iion of tendrils may be safely admitted, as shown by 

 the recent researches of H. de Vries. I will, however, 

 add one little fact in support of this conclusion. 



If the short, nearly straight portion of an attached 

 tendril of Passiflora gracilis, (and, as I believe, of other 

 tendrils,) between the opposed spires, be examined, it 

 will be found to be transversely wrinHed in a con- 

 spicuous manner on the outside ; and this would 

 naturally follow if the outer side had grown more than 

 the inner side, this part being at the same time 

 forcibly prevented from becoming curved. So again 

 the whole outer surface of a spirally wound tendril 

 becomes wrinkled if it be pulled straight. Nevertheless, 

 as the contraction travels from the extremity of a 

 tendril, after it has been stimulated by contact with a 

 support, down to the base, I cannot avoid doubting, 

 from reasons presently to be given, whether the whole 

 effect ought to be attributed to growth. An unattached 

 tendril rolls itself up into a flat helix, as in the case of 

 Cardiospermum, if the contraction commences at the 

 extremity and is quite regular ; but if the continued 

 growth of the outer surface is a little lateral, or if the 

 process begins near the base, the terminal portion can- 

 not be rolled up within the basal portion, and the 



