160 TENDRIL-BEARERS. -Cuar. IV. 
thin twigs, become deeply sinuous or zigzag. More- 
over 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- 
tion 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 wrinkled 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 
