IRRITABILITY. 5 1 1 



Dutrochet's is the only one which suggests that twining 

 stems possess properties which are peculiar to themselves, 

 and are not possessed by stems which do not twine. He 

 attributes to them, namely, an inherent tendency to grow 

 spirally. But, even admitting this, this assumption does not 

 explain the twining of these stems round supports. For it is 

 clear that every stem which exhibits torsion must have been 

 growing spirally ; that is, its apex has been circumnutating 

 and at the same time a twisting round its own axis has been 

 taking place, conditions which necessitate a spiral direction of 

 growth. We must, to be consistent, admit that every stem 

 which exhibits torsion has an inherent tendency to grow 

 spirally. And yet it is not true that all stems which undergo 

 torsion are capable of twining. It is the confusion of a spiral 

 direction of growth with twining round a support which 

 makes all these explanations unsatisfactory. All stems which 

 twine grow spirally, but the converse is not true, that all 

 stems which grow spirally twine. It is true that, as de Vries 

 pointed out, anything which interferes with the circumnu- 

 tation of a twining stem induces spiral growth, but this is not 

 true only of the stems of twining plants, it is true of all cir- 

 cumnutating stems whatsoever. 



Von Mohl recognised this difficulty, and met it by 

 assuming that twining stems must be endowed with irri- 

 tability. This view was contradicted by all subsequent 

 observers, until, quite recently, Kohl reasserted it, as already 

 mentioned above, and he has proved it in various ways. 

 His most striking observation is perhaps this, that a stem 

 of Calystegia will twine round a perfectly loose string. It 

 is clear that the string cannot mechanically interfere with 

 the circumnutation so as to cause the stem to grow spirally 

 round it ; and yet the stem was concave at all the points of 

 contact. It is impossible to explain this otherwise than Kohl 

 has done, that is, by assuming that the stem is irritable, like 

 a tendril, though perhaps in a less degree, and that, as in the 

 case of a tendril, the concavity at each point of contact is the 

 result of the contact upon the irritable organ. When we 

 remember that the stem of one twining plant, Cuscuta 



