194 MODIFICATIONS OF BARK. [BOOK i. 



rapidity and in great quantities. It does not appear to have 

 any communication by lateral passages with the interior of 

 the plant ; although Dutrochet represents them to exist in 

 Ulmus suberosa, where I cannot find them. After a certain 

 age, it exfoliates, in the Cork Tree, but in such plants as the 

 Maple (Acer campestre), the Elm, &c., it is simply rent and 

 thrown off piecemeal. In the Birch, the Cherry, and similar 

 trees, it forms annually only a few layers of tabular cellular 

 tissue, arranged in transverse rows, which separate at a certain 

 age into thin silvery lamellae : these have been improperly 

 confounded with the epidermis. The cause of the separation 

 of the lamellae of the epiphlceum of the Birch is found in the 

 development, between the lamellae, of a layer of thin-sided 

 cells, less compactly arranged, and easily separating into a 

 fine powder when disturbed. 



As strata of cellular tissue, in a peculiar state, may form 

 between the lamellae of the Birch and other such trees, so may 

 it in other parts of the bark. This causes the sloughing of 

 hard thin plates from the bark of the Plane tree ; which Mohl 

 explains thus : Up to its eighth or tenth year, the bark of 

 the Plane tree is like that of the Beech ; at that period there 

 forms in different parts of the liber a stratum of tabular cells, 

 in all respects analogous to those of the epiphloeum. This 

 new epiphloeum is not exactly parallel with that of older 

 date, which exists at the surface of the bark, and cuts off an 

 exterior portion, which then dies and drops off in the manner 

 with which we are all familiar. The scales produced by this 

 formation of epiphloeum inside the liber or mesophloeum 

 Mohl calls Rhytidoma, from prm?, a wrinkle. (Ann. des 

 Sciences, N. S. ix. 290.) 



In some plants the epiphloeum forms regular strata, parallel 

 with the axis of the stem, and afterwards separates into strips 

 analogous to those of the liber, as in the Juniper, Callistemon 

 lophanthus, &c. In others, a portion of the liber is really 

 thrown off annually, as in the Vine, the Honeysuckle, &c. 



Hence in exogenous trees, the thickness of the bark is 

 annually diminished by one of two causes ; either by an exfo- 

 liation of the external and dead portions of the epiphloeum 

 only, or by a formation of a second epiphloeum, or false cork, 



