364 CONCLUDING REMAEKS Chap. XV. 



number of the most widely distinct orders, — in the 

 leaves of the many plants which go to sleep at night, 

 or move when shaken, — and in the irritable stamens 

 and pistils of not a few species. We may therefore 

 infer that the power of movement can be by some 

 means readily acquired. Such movements imply irri- 

 tability or sensitiveness, but, as Cohn has remarked,* 

 the tissues of the plants thus endowed do not differ 

 in any uniform manner from those of ordinary plants ; 

 it is therefore probable that all leaves are to a slight 

 degree irritable. Even if an insect alights on a leaf, 

 a slight molecular change is probably transmitted 

 to some distance across its tissue, with the sole 

 difference that no perceptible effect is produced. We 

 have some evidence in favour of this belief, for we 

 know that a single touch on the glands of Drosera does 

 not excite inflection ; yet it must produce some effect, 

 for if the glands have been immersed in a solution of 

 camphor, inflection follows within a shorter time than 

 w^ould have followed from the effects of camphor 

 alone. So again with Dionaea, the blades in their 

 ordinary state may be roughly touched without their 

 closing ; yet some effect must be thus caused and 

 transmitted across the whole leaf, for if the glands have 

 recently absorbed animal matter, even a delicate touch 

 causes them to close instantly. On the whole we may 

 conclude that the acquirement of a high degree of 

 sensitiveness and of the power of movement by certain 

 genera of the Droseraceae presents no greater difiiculty 

 than that presented by the similar but feebler powers 

 of a multitude of other plants. 



* See the abstract of his me- Map;, of Nat. Hist.' 3rd series, 

 moir on the contractile tissues vol. xi. p. 188. 

 of plants, in the ' Annals and 



