64 
J. SAUNDERS—CLIMBING PLANTS 
line from right to left for several feet on the stem of the sup¬ 
porting plant. As the extra deposition of woody tissue was above 
the line of contact, the inference is that the formative material 
was arrested in its downward flow. 
Familiar examples of twining stems that turn from left to right 
are furnished by the scarlet-runners ( PJiaseolus ) of cultivation, and 
the bindweeds, including the field convolvulus and the large white 
hedge convolvulus, both of which are common in our neighbour¬ 
hood. In the latter the rootstocks are underground, and as the 
stems are annual and slender they cause hut little injury to the 
plants around which they climb. The field convolvulus is, however, 
detrimental to cereals. 
Associated with the Convolvuli, of which they are specialized 
offshoots, are the leafless, parasitic dodders. These are annuals, 
and attach themselves to various herbaceous and shrubby plants 
from which they draw nourishment by means of sucker-like organs 
(hausteria ) which are developed on the stems and assume the 
functions of roots. Although so closely allied to the Convolvuli , 
they have diverged from the habit of that genus of turning from 
left to right. The dodders have no definite mode of growth, 
turning, twisting, and doubling hack in every direction, for which 
habit the most descriptive term is “ scramble.” Wherever food 
can he obtained with least effort, to that point growth is directed. 
The species that occur in the South Midlands are: Cuscuta trifolii, 
which is sporadic in clover-fields and is too frequently introduced 
with foreign seeds; C. Hpithymum, which occurs occasionally on 
sandy heaths attached to ling, thyme, etc.; and C. europea , which 
is rare with us, growing on nettles, etc. 
In the common ivy ( Hedera Helix ) the stems develop a great 
number of adventitious roots by which the plant adheres firmly to 
any means of support that is available, such as walls, fences, and 
living trees. When it attains a vigorous growth on deciduous 
trees, its perennial foliage checks the development of the leaves of 
the host plant. It is also probable that the constriction caused by 
the adhesion of the stems and branches of the ivy retards the flow 
of the sap of the tree to which it is attached, and hence diminishes 
the supply of nutritive material to the growing extremities. 
The twining habit of the common ivy occurs occasionally in 
ancient hedgerows which have not been cut down for forty or fifty 
years. An illustration of this exists at Chaul End, on the borders 
of the adjoining counties of Beds and Herts. There are several 
instances of ivy twining round stems and branches of blackthorn, 
hazel, and maple. The twining is indiscriminate, sometimes com¬ 
plicated, and occasionally becoming reversed in the same axis. In 
one instance, associated with blackthorn, an ivy stem 1£ inch in 
diameter accomplished two complete spirals from right to left, on 
the main axis, which was followed by another in the opposite 
direction on a short lateral branch. (See Fig. 20, p. 65.) The 
twining is most pronounced just below the fruiting branches, 
presumably to give increased support to these organs. The ivy 
