CLIMBING PLANTS. 
217 
while every single plant in a row of beans twines the other 
way ; nor why in some rare instances a species is divided, like 
the human race, into right and left-handed individuals, some 
twining like a left-handed, others like a right-handed screw. 
Or, again, why some very few plants will twine half-way up 
a stick in one direction, and then reverse the spiral and wind 
the other way. Nor though we know that in all these plants 
the twining is caused by the change in the region of quickest 
growth, have we any idea what causes this change of growth. 
There is still much to work at, and it is to be hoped that there 
are still plenty of workers to solve the problems. It is 
by looking to exceptions that the key to a problem is often 
found. It is the exceptions to general rules that often lead us 
to understand the meaning and origin of the rules themselves ; 
and it is to such exceptions that any one who wants to work 
at climbing plants should turn. Now, it is a general rule that 
a climbing plant twines in the same way that it revolves. It 
seems an obvious thing that in the case of the rope model, if 
we swing the rope round our head in the direction of the hands 
of a watch, it must twine round the stick against which it 
strikes in the same direction. But in plants it is not always 
so. In the large majority of cases it is so, for if this were 
not the case, the illustration of the rope would not have been 
applicable ; but it is not universally the rule. Every individual 
of the plant Hibbertia always twines round its stick in the same 
direction, but when it is performing the swinging movement in 
search of a support, it is found that some plants travel round 
with the sun, others in the opposite direction. This fact forms 
an exception of a striking kind — and such exceptions are 
worthy of close study. 
There are other facts of a different nature, which seem to 
show how difficult the problem is, and how delicately balanced 
is that part of the organization of the plant which is con- 
nected with the power of climbing. For instance, if we cut a 
branch of most shrubs, and put it in water, it goes on growing, 
apparently as healthily as ever. Indeed the practice of making 
cuttings — where a cut-off branch or shoot developes roots and 
turns into a new plant — shows us that no serious injury is 
thus caused. But the twining organization is sensitive to 
such treatment. A cut branch of hop placed in water was ob- 
served to make its revolutions in about twenty hours, whereas 
in its natural condition — growing on the plant — it makes a 
complete turn in two or three hours. Again, if a plant growing 
in a pot is moved from one green-house to another, the slight 
shaking thus caused is sufficient to stop the revolving move- 
ment for a time, — another proof of the delicacy of the internal 
machinery of the plant. 
