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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 w r hich 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 

 generally 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 i), 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." 



