Chap. XVL] MOVEMENTS OP THE LEAVES. 307 



glands of which all round the insects were no doubt secret- 

 ing. We can thus also understand how it is that so many 

 insects, and fragments of insects, are generally found lying 

 within the incurved margins of the leaves. 



The incurvation of the margin, due to the presence of an 

 exciting object, must be serviceable in another and probably 

 more important way. We have seen that when large bits of 

 meat, or of sponge soaked in the juice of meat, were placed 

 on a leaf, the margin was not able to embrace them, but, as 

 it became incurved, pushed them very slowly towards the 

 middle of the leaf, to a distance from the outside of fully .1 

 of an inch (2.54 mm.), that is, across between one- third and 

 one-fourth of the space between the edge and the midrib. 

 Any object, such as a moderately sized insect, would thus be 

 brought slowly into contact with a far larger number of 

 glands, including much more secretion and absorption, than 

 would otherwise have been the case. That this would be 

 highly serviceable to the plant, we may infer from the fact 

 that Drosera has acquired highly developed powers of move- 

 ment, merely for the sake of bringing all its glands into 

 contact with captured insects. So again, after a leaf of Di- 

 onsea has caught an insect, the slow pressing together of 

 the two lobes serves merely to bring the glands on both sides 

 into contact ^th it, causing also the secretion charged with 

 animal matter to spread by capillary attraction over the 

 whole surface. In the case of Pinguicula, as soon as an 

 insect has been pushed for some little distance towards the 

 midrib, immediate re-expansion would be beneficial, as the 

 margins could not capture fresh prey until they were un- 

 folded. The service rendered by this pushing action, as 

 well as that from the marginal glands being brought into 

 contact for a short time with the upper surfaces of minute 

 captured insects, may perhaps account for the peculiar 

 movements of the leaves: otherwise, we must look at 

 these movements as a remnant of a more highly devel- 

 oped power formerly possessed by the progenitors of the 

 genus. 



In the four British species, and, as I hear from Prof. 

 Dyer, in most or all the species of the genus, the edges of 

 the leaves are in some degree naturally and permanently in- 

 curved. This incurvation serves, as already shown, to pre- 

 21 



