18 
HOW PLANTS CLIMB. 
which they avoid winding themselves around the stem they belong to. The ac- 
tive tendrils are of course near the top of the stem or branch. The growing 
summit beyond the tendril now seeking a support is often turned over to one side, 
so that the tendril, revolving almost horizontally, has a clear sweep above it. But 
as the growing stem lengthens and rises, the tendril might strike against it and be 
wound up around it. It never does. If we watch these slender Passion-flowers, 
which show the revolving so well in a sultry day, we may see, with wonder, that 
when a tendril, sweeping horizontally, comes round so that its base nears the 
parent stem rising above it, it stops short, rises stiffly upright, moves on in this 
position until it passes by the stem, then rapidly comes down again to the horizon- 
tal position, and moves on so until it again approaches and again avoids the 
impending obstacle ! 
29. Other equally curious illustrations might be given ; but these may serve 
the purpose of opening the eyes to what is going on around us, awaken an intel- 
ligent interest, and excite to further observation. They are enough to make it 
clear that the two vegetable prodigies described at the beginning of this chapter, 
surprising as they are, have no peculiar endowments. Climbing plants generally, 
and tendril-climbers especially, exhibit both the free movements of the one, and 
the movement in response to external irritation of the other. The sweeping round 
of tendrils is like the movement of the leaflets of Desmodium gyrans : their coil- 
ing upon contact, and the similar coiling of some leafstalks, are to be compared 
with the movement of the leaflets and leafstalks of the Sensitive Plant. 
30. This becomes evident when the motion is quick enough to be seen by the 
eye. It has already been stated that a very long tendril of one of the slender 
Passion-flowers has often been seen to move. Still oftener may it be seen to coil 
up at the tip when gently rubbed. This is also to be seen in the Bur-Cucumber 
(Sicyos), a common weed of the Gourd Family. When, in a sultry summer day, 
we gently rub, with a stick or with the finger, the upper end of a vigorous tendril, 
it will respond within half a minute by coiling up so rapidly that the motion may 
be distinctly seen. It will soon straighten, but will coil again if the rubbing is 
repeated. If a stick be left in contact the coiling will be permanent ; and a 
downward propagation of the same action is what throws the whole tendril into 
spiral coils. 
