164 Explanalion of the CiUicla of Leavet. 



scope, which if well managed throws llie cuticles to their focal 

 distance, showing the cylinders of the hairs between, and thus 

 dividing the cuticles when they are too close for either lancet 

 or finger to do it. It mav be observed of all leaves, that tiie 

 young ones have much more hairs thim the old, and I may say 

 that it is difficult to find a plant even of trees when the buds ar,e 

 in their ag?,regate state, whose leaves are not loaded with hairs, 

 and often covered with a sort of net of that kind, which entirely 

 disapjjears in the more advanced age of the plant : nor does less 

 alteration take place in the edge of young leaves, which ge- 

 nerally change in a very extraordinary niainicr. This is not to 

 be seen by the naked eye, but is very conspicuous to the dissec- 

 tor, who, in all these clianges, finds one general system running 

 throughout nature^ and tending to j)rocure to the young elcve 

 plenty of nourishment, (sec fig. III. The sides of ycung leaves, 

 when compared with old ones of the same plant.) There is 

 also another peculiarity in which the leaves of trees differ much 

 from those of herbaceous plants ; the pabulum is not only more 

 solid, but their cuticles lie much closer to each other, and the 

 older the leaves grow, the more perceptible is this circumstance. 



In herbaceous and annual plants, and indeed all those which 

 rise each year from the earth, the pabulum is less solid than in 

 the leaf of trees, and more watery; they have infinitely more 

 hairs, and therefore receive much more moisture from the at- 

 mosphere; but they have in general but One set of veins, which 

 lie next the under cuticle, and cause it to be much more raised 

 and pulpy in its upper surface than the leaf of trees : it is this 

 that gives the leaves of the melissa and menthu that peculiar look. 

 It is this also which gives the cabbage the same; and it is 

 merely the drawing in of the spiral wire which produces in the 

 hrassica that curious effect on a cold morning. I have taken it 

 M'ithin doors very often, and seen it almost smooth; when return- 

 ing it tt» the frost, it has directly shrunk up into absolute little 

 nests, by means of the contraction of the spiral wire in the se- 

 veral divisions which are drawn in like a high quiltirig. 



I have in vain tried to discover the reason for these leaves 

 having but single veins, as I doubt not some curious cause is 

 attached to it ; but it requires much time to develop all these 

 peculiarities and minute differences, and watching atone can dis- 

 cover them. 



I gave in mv last letter a specimen of the sort of cuticle of 

 hairs that often covers annuals and herbaceous plants, that will 

 contivmally spread over both the upper and under surfaces of the 

 leaves, and that had a bottle attached to it with a curling vessel 

 tv.-isting hke the spiral wire. Many different sorts might be 

 giveii as curious as that : it is inconceivable how much moisture 



they 



