CLIMBING PLANTS AND PARASITES 47 



in the case of the twining stem. One side grows 

 faster than the other, and so the stem behaves in the 

 same way as the dandelion stem, but more slowly. 

 Often, as in the Victorian climbing lilac, several stems 

 twine together and so are able, without other support, 

 to raise their flowers much higher into the air than a 

 single stem could. Some plants, again, like the 

 clematis, use their leaf stalks for twining. 



5. Twining stems buried in wood. Have you 

 ever noticed a strong wire that has been twisted round 

 a growing tree ; how it sinks deeper and deeper into 

 the tree as the years pass ; and how it may even be 

 buried in the wood ? The same thing often happens 

 to the woody stems of climbing plants. Think what 

 happens. The root-sap is not checked in its upward 

 flow by the wire, but when the leaf-sap flows down 

 through the inner bark it is stopped when it reaches 

 the wire. This check of the leaf- sap flow causes a strong 

 growth of wood and bark just over the wire ; for, as 

 you remember, the leaf- sap is full of material for 

 building and repairing. In this way, the upper lip 

 may, in time, meet the lower lip of the wound, and 

 then the wire is completely covered. After this, the 

 tree grows freely again, and, in a few years, the wire 

 is deep in the dead heartwood. But sometimes, the 

 lips fail to join over the wire or the twining 

 stem, and in this case the plant becomes feeble or 

 even dies. Just as the folds of a great snake can 

 choke an animal, so the rings of the climber may 

 squeeze the life out of the plant. 



6. Next, we come to plants that climb by tendrils. 

 Look at a Virginia creeper, and you will see that a 

 little branch has been changed into a kind of hand for 



