OF THE SOUTH MIDLANDS. 
65 
branches that assume the twining habit, so far as observed, vary in 
thickness from £ inch to 1A- inches, and the stems of the various 
species to which they are attached range from an inch to 5 or 6 
inches in diameter. 
The largest stem, that of maple, around which ivy was seen to 
twine, did not exceed 6 or 7 inches in thickness. In the immediate 
vicinity were tall trees of beech, ash, and oak, from 18 inches in 
diameter and upwards, but in all these instances the ivy that grew 
on them climbed without twining. 
Fig. 20. —Ivy twining round blackthorn at Chaul End, Beds (Feb. 1904), 
one-sixth natural size. Most of the leaves of the ivy have been cut away. 
It is a curious fact that twining stems fail to attach themselves 
to supports that are too large for the purpose; the maximum size 
appears to be determined by the dimensions of the coil which the 
stem can produce without such assistance. 
In asparagus the main axis, when young, possesses the power of 
twining. Some exotic species of this genus have true twining 
stems. (Brown’s Botany, p. 582.) 
Petioles .—The British species that climb by means of petioles 
are fewer than those that do so either by the stem or by tendrils. 
A well-known example of this method is furnished by the wild 
clematis, which is so abundant over the Chalk district. The 
curvature of the petiole is induced by contact with any means of 
support; those leaves that do not receive this stimulus have the 
VOL. XII.—PART II. 
5 
