THE STEM 99 
cucumber climb by means of tendrils, sensitive, 
coiling, wire-like modifications of various organs in 
which a marked irritability of the protoplasm shows 
itself in their response tc the contact of adjacent 
objects. In the passion flower (Fig. 68), the tendril 
is a modified branch, as may be seen from the fact that 
it arises in the axil of a leaf; in the vine (Fig. 69) it 
is a modified inflorescence, indicated by the cireum- 
stance that it may on occasion bear a few flowers; in 
the cucumber (Fig. 67) the tendrils are adapted from 
the stipules, while in the pea (Fig. 65) they are formed 
from the midribs of leaflets that have lost their flat 
ereen blades. The Virginia creeper (Fig. 66) climbs 
by its branching tendrils which are produced like 
leaves. At the tip of each tendril-branch is formed a 
dise-like sucker that attaches the plant to walls and 
other flat objects. Tendrils are highly sensitive, and 
respond to the contact of a swinging object less than 
an eighth of the weight of the lightest object that can 
be felt by the most sensitive part of the human skin. 
With all twiners, as well as leaf and tendril climbers, 
the process is the same. The development of the side 
that is touched is checked, while development con- 
tinues on the side remote from the object with which 
the tendril is in contact. This causes the organ to 
eurve spirally round the support till it is firmly 
fastened to it. 
Epiphytes are here mentioned in order that they 
may not be confused with climbers. Epiphytes are 
plants that grow on other plants. They differ from 
parasites in not penetrating with their roots the tissues 
of the plant on which they live. While parasites send 
their roots into and live on the sap of the host, 
epiphytes gain their food materials from the decom- 
posing bark, dust, and other substances that have fallen 
in the fork or stuck to the trunk of the tree on which 
they live. Thus the mistletoe, which, in its season, 
