CLIMBING PLANTS. 639 



left-handed screw, 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 twin- 

 ing 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 direc- 

 tion, 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 Illbbertia 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 connected with the power of climb- 

 ing. 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 

 develops 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 observed 

 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 

 greenhouse to another, the slight shaking thus caused is sufficient to 

 stop the revolving movement for a time another proof of the deli- 

 cacy of the internal machinery of the plant. 



Some of the problems, as, for instance, why twining plants can not 

 as a rule climb thick stems, may be looked at from the natural-history 

 point of view. Most of our climbing plants die down in the winter, 

 so that, if they were able to climb round big tree-trunks, they would 



