£72 



What is the 

 the final cause 

 of this power? 



Darwin sup- 

 poses it intend- 

 ed to keep off 

 insects. 



More probably 

 fbrth<? purpose 

 of catching 

 the ih. 



Leaves of the 

 drosera well 

 adapted for 



this* 



ON THE IRRITABILITY OF VEGETABLES. 



" tion of the phenomena mentioned". (Introduct. to Bo- 

 tany.) 



Having found, that the leaves of the'drosera catch flies ou 

 the principle of irritability, it may be afked, what is the in- 

 tention of nature in allowing these and a few others that ex- 

 clusive power ? At present I am afraid this question can- 

 not be satisfactorily answered; but in pursuing the inquiry 

 we ought first to fix whether the particular contrivances in 

 the dionoea, drosera, &c, are intended for offence or de- 

 fence. Darwin entertained the latter idea; for, after men- 

 tioning the silene, he says: "In the dioncea muscipula 

 " there is a still more wonderful contrivance, to prevent the 

 " depredations of insects." Again, when speaking of the 

 drosevae, he remarks, that, " This mucus is a secretion from 

 " certain glands, and, like the viscous material round the 

 " flower stalks of silene (catchfly), prevents small insects 

 " from infesting the leaves." 



I should rather be disposed to think, that the leaves of the 

 dioncea and droserae were intended for offence, i.e. for catch- 

 ing flies; for if defence was wanted, nature, ever simple in 

 her operations, cauld have supplied these plants with a 

 much simpler apparatus, as a number of spines, which would 

 have been quite sufficient for this purpose; while we can- 

 not conceive any contrivance, that would have answered bet- 

 ter for catching flies, than what is seen in their leaves. 

 Reasoning from analogy, this position will be strengthened. 

 The sarracenia purpurea has tubular leaves beset at the 

 margin with inverted hairs, which, like the wires of a mouse 

 trap, render it very difficult for any unfortunate fly, that 

 has fallen into the watery tube, to crawl out again. Now 

 had it been the intention of nature, that this contrivance 

 was for defence — would it not have been much easier for 

 her to have placed the hairs on the margin of the leaf with 

 the points upwards, instead of inverted, which would have 

 effectually prevented the insect even from touching the in- 

 side of the leaf? 



Regarding then these contrivances for offence, we have 

 found the structure of the leaves of the droserae admirably 

 well calculated for this purpose. The glands are covered 



with 



