658 Microscopical Essays. 



eflays ; by this means, together with the microfcope, we find that 

 the vefTels are every-where pierced with fmall dots, or openings ; 

 of the ufe of thefe the following conjectures have been formed.* 



The root, which is equal in furface to a third part of the tree 

 above ground, is covered with a pierced rind. The cold of 

 winter contracts the whole of this, the parts are drawn clofer to- 

 gether, and the mouths of thefe innumerable veflels are (hut, or 

 nearly fo, by this contraction ; a very little of the half congealed 

 moiflure of the ground gets into them, but this fuffices for the 

 fervice of the tree, when there is little heat to caufe any perfpira- 

 tion, and at a time when in the deciduous trees, the very organs 

 of the greater! perfpiration, the leaves, do not exift. 



The warmth of fpring arrives, the fluids of the earth grow 

 thinner, every part of the root expands ; this opens the mouths of 

 the veflels, and the torrent of nutrition rufhes in. By this means, 

 every coat of the rind, and the interfiitial fpaces thereof, are ren- 

 dered fupple, and may be eafily feparated from the under 

 coverings. 



In roots the colour of the rind varies very much, being white 

 in fome, brown in others, &c. Every root, according to Grew, 

 after it has arrived at a certain age, has a double fkin. the one 

 coeval with the other parts,, and ex ills in the feed : a ring is after- 

 wards (em off from the bark, which forms the fecond flan ; thus 

 in the root of dandelion, towards the end of May, the original or 

 outer fkin appears mrivelkd,. and is eafily feparated from the new 



one,. 



* Hill's Conftruaion of Timber, &c. p. 37* 



