1Q 
of the top of the stem causes it to grow round any upright 
or nearly vertical object in its neighbourhood, and then 
as the slender stem tries to straighten itself out it grips 
firmly the support round which it has grown. As the 
rotation is always in the same direction for a given species, 
it is necessary before giving such a plant any artificial 
aid to see in which way the rotation takes place, for if one 
twines the plant in the wrong direction it will unwind 
itself again when left alone. It is difficult to keep a 
twining plant to a horizontal course as it of itself will 
never twine round a horizontal support. Climbing plants 
on the other hand can fix their special climbing organs or 
tendrils to horizontal supports, partly owing to the sensi- 
tiveness of these organs to contact. In some cases, as for 
instance in the Virginia Creeper (Ampelopsis) the contact 
stimulus causes the tips of the tendrils to swell up into 
sticky suckers, which enable this creeper to fix itself to 
a vertical wall. More frequently however when the ten- 
drils come into touch with some object, they grow round 
it by virtue of the fact that the side which is touched 
grows less rapidly, so that it curves round the support. 
After it has grasped the latter the tendril contracts spirally, 
and thus tightens the climber to its support. Tendrils are 
zenerally formed from parts of the leaf, the leaf-tips in 
peas, the leaf-stalks in the Nasturtium and Clematis.* 
Besides carrying the leaves the stem has the further 
function of conducting to these leaves the food material 
absorbed by the roots. As we have seen the forces at 
work in connection with the supply of water to the leaves 
are, firstly, the root pressure (lecture 2), and secondly, the 
transpiration current in leaves (lecture 1), which latter 
causes a considerable suction to be exerted on the water 
in the stem. The special channels through which the sap 
rises are the vessels, long continuous passages, running in 
the wood of the stem. But this upward passage of water 
with the inorganic salts it contains, is not the only con- 
duction required. The complex substances formed, as 
we shall see, in the leaves require to be conducted away 
partly to nourish the flowers and fruits, partly to enable 
further growth and development of the roots to take place. 
This elaborated sap passes through other channels which 
* Further information concerning this interesting group of 
plants can be obtained by the perusal of Charles Darwin’s 
*“ Movements and Habits of Climbing Plants.” 
