Chap. XV.] ON THE DROSERACE^. 295 



which go to sleep at night, or move when shaken, and in 

 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 irritability 

 or sensitiveness, but, as Cohn has remarked," the tissues of 

 the plants thus endowed do not differ in any uniform man- 

 ner 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 in- 

 flection; yet it must produce some effect, for if the glands 

 have been immersed in a solution of camphor, inflection fol- 

 lows within a shorter time than would have followed from 

 the effects of camphor alone. So again with Dionaea, the 

 blades in their ordinary state may be roughly touched with- 

 out their closing; yet some effect must be thus caused and 

 transmitted across the whole leaf, for if the glands have re- 

 cently 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 diflficulty than that presented by the 

 similar but feebler powers of a multitude of other plants. 



The specialised nature of the sensitiveness possessed by 

 Drosera and Dionaa, and by certain other plants, well de- 

 serves attention. A gland of Drosera may be forcibly hit 

 once, twice, or even thrice, without any effect being produced, 

 whilst the continued pressure of an extremely minute par- 

 ticle excites movement. On the other hand, a particle many 

 times heavier may be gently laid on one of the filaments of 

 Dionsea with no effect ; but if touched only once by the slow 

 movement of a delicate hair, the lobes close; and this differ- 

 ence in the nature of the sensitiveness of these two plants 

 stands in manifest adaptation to their manner of capturing 

 insects. So does the fact, that when the central glands of 

 Drosera absorb nitrogenous matter, they transmit a motor 



"See. the nbstrnct of his me- of \nt. Hist.' 3rd series, vol. xl. 

 molr on the contrnctlle tissues of p. 188. 

 plants, In the ' Annals and Mag. 



