14 TRANSACTIONS LIVEEPOOL BIOLOGICAL SOCIETY. 



be a far more unstable guide when the function is both 

 more general and more varied. This last is the case when 

 we have to deal with a plurality of functions, any one of 

 which may be in the ascendant in a particular part of the 

 tissue without any clear structural reason to hang that 

 ascendancy on. 



Let me remind you of one typical example. In the 

 Dionsea muscipulata, the two lobes of the leaf are 

 sensitive to mechanical contact and other stimuli. "When 

 touched, they rapidly close and imprison, as in a dungeon 

 with barred gates, any object which is in contact with 

 their inner surfaces. This movement is produced when 

 the leaf is rudely touched, but there is one part which is 

 exquisitely sensitive to the lightest touch ; this part is in 

 the middle of the leaf in the immediate neighbourhood of 

 three hinged hairs, and is termed the trigone. 



A fly walking over the leaf does not excite the closure 

 of the lobes until it reaches the central trigone, or strikes 

 these so-called sensitive hairs. The use of this to the 

 plant is plain on the assumption that it is desirable for it 

 to have an occassional meal of animal food ; for the fly is 

 so far in that it cannot get out before the gates close, and 

 implanted in its prison it is slowly digested by the peptic 

 juices which are poured out upon it by little glands on the 

 inner surface of each leafy wall. 



Surely with such an apparently elaborate function one 

 might expect that the structure of the cells in this sensitive 

 part would be different to the structure elsewhere. 

 Histological investigation, however, shows no difference 

 corresponding with the ascendancy of the function. The 

 sensitiveness (or excitability) is but one of a plurality of 

 functions possessed by the cells (power of retaining liquids ; 

 loss of this power on stimulation, electrical changes, etc.,) 

 and is the common property of all the cells in the leaf, but 



