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POPULAR SCIENCE REVIEW. 
and this is probably the most important one. Thus use depends 
on the fact that a contracted tendril acts like a spiral spring, 
and is thus converted into a yielding, instead of an unyield- 
ing, body. The spirally-wound tendril yields like an elastic 
thread to a pull which would break the tendril in its original 
condition. The meaning of this arrangement is to enable 
the plant to weather a gale which would tear it from its 
support by snapping the tendrils, if they were not converted 
into spiral springs. 
My father describes how he went in a gale of wind to 
watch the bryony on an exposed hedge, and how, in spite of 
the violent wind which tossed the branches of the plant about, 
the bryony safely rode out the gale, ‘like a ship with two 
anchors down, and with a long range of cable a-head, to serve 
as a spring as she surges to the storm.’ It may also serve to 
divide the weight which has to be supported equally among a 
number of tendrils ; and this is the meaning of the spiral con- 
traction seen in the tendrils of the Virginia Creeper. 
It can be seen in Fig. 4 that all the coils of the spiral 
are not in the same direction. First, there are two in one 
direction, then six in the other, and then three again in the 
first direction, making six turns in one way and five in the 
other. And this is universally the case; the turns in one 
direction are always approximately equal in number to those in 
the opposite direction. It can be shown to be a mechanical 
necessity that a tendril which has its two ends fixed, and 
which then coils into a spiral, should behave in this way. 
A simple model made to show this mechanical necessity is 
described by Sachs in his Text Book. It is made by stretching 
a strip of indiarubber and cementing it to an unstretched 
strip. The strips being united in a state of longitudinal strain, 
form a spiral when released. If the model is held by one end 
only, the turns of the spiral are all in one direction. And 
this represents the behaviour of a tendril which has not 
managed to seize a support; for some unknown reason such 
tendrils contract into spirals, and the turns of such spirals 
are all in one direction. But if the india-rubber is held at 
both ends, half the turns are in one direction, half in the 
other, just as with a tendril the same thing happens. 
Now let us consider the general relations that exist between 
twining plants, leaf-climbing plants, and tendril- climbing 
plants. To an evolutionist the question how these various 
classes of climbing plants have been developed is perhaps of 
most interest. What is the relationship between them ? Have 
all classes been developed separately from ordinary non-climb- 
ing plants, or has one class been developed out of one of 
the others ; and if so, which is the oldest form of climbing 
