ll£ CAUSE OF MOTION IN PLANTS. 



is of so thin a substance, or rather I believe I should say of 

 So loose a one, as plainly to be intended to dilate and con- 

 tract; a few very thin vessels, interlaced with an extremely 

 fine spiral wire, composes it, as will be seen in the plate: 

 while the larger spiral vessels fill up the case in an irregular 

 manner, the nourishing- vessels forming a regular circle of 

 Spiral wire tubes round it, and, to complete the contrivance, the midrib 

 contacting or of ^ ]eaf • f ovrne( j a | so to con tract and dilate a little with 

 dilating. 



perfect ease, even in the hardest leaves, as the laurel, &c. 



But in some leaves there is a curious contrivance to lengthen 

 it in the bosom of the leaves just where the bud is concealed 

 Mid r.b of the in \ts first birth, as in the ash, plane, &c. The peach, and 

 stretches 3 most of that order, will stretch the midrib of the leaf far 



more than is necessary for any succeeding motion : and if 

 any person could doubt the power of the spiral wire to draw 

 up, and of course turn the leaf, he has but to take a leaf 

 so drawn from a nectarine, or peach ; for it seems impossi- 

 ble that it should not be seen, that it is drawn from the in- 

 side of the midrib, and not from the gathering up of the 

 cuticle of the leaf, which has been suggested. I have seen 

 in the geranium the spiral wire to have stretched the case 

 with such violence, it could not return to its usual size, but 

 has remained in a spiral form : which shows however how 

 easily the spiral wire acts on the case. 

 Different parts ^ ma y ^ e thought, tnat lt was not necessary the stalk 

 stretch in dif- should dilate itself, provided the part within did ; but na- 

 ferent degrees. tuj>e g^ its account in this arrangement, the leafstalk 

 could not turn with ease, did not one side of it contract, t© 

 wind it round. They have alt in this respect their appropriate 

 proportions, the spiral wire stretches to any degree wanted, 

 the case a great deal less than the former, and the outward 

 cuticle has only flexibility enough not to impede the con- 

 tinual irritability of its inward spiral wire. 

 Extrem- effect In my first letter on this subject, I showed the effect' of 

 of the spiral the spiral wire on plants in general, selecting those that 

 were only commonly affected by it, that I might not be ac- 

 cused of favouring my subject, in order to conceal any 

 weakness the argument might possess. It is truth alone I 

 teek, and I can have no other attachment to it but sup- 

 ptwh g it 60 ; my eyes and my microscope must grossly de- 

 ceive 



