56 Prof. Meyen*s Report of the Progress of 



bark consist of the cellular envelope and the layer of liber : the 

 intermediate parenchymatous cells are very thick-sided. (See 

 the figures of the development of the birch bark which Link 

 has given in the Icon, Anat. Bot. Tab. vi. fig. 12, 14-, and 15.) 



In the very thick bark of old birch stems the afore-mentioned 

 regularity in the position of the brown and white layers is not 

 observable ; but the increase in thickness takes place here and 

 there in a higher or lower degree, by which the previous per- 

 fectly regular concentric laminae are bent and torn in various 

 ways. 



We have already mentioned those cases which show that 

 the distinct development of the bark consists sometimes in the 

 thickening of the cork substance, at others in the thicken- 

 ing of the cellular envelope ; there are however many cases in 

 which the great development of the bark substance consists 

 chiefly in the development of the layer of liber ; we may cite 

 for instance the beech (Fagus sylvatica). In this tree the 

 bark almost always remains even ; the cellular envelope here 

 always remains very small, even when the bark has become 

 of considerable thickness. 



The bark also of the plane-tree {Platanus occidentalis) which 

 is found in this country must also be specially mentioned. It 

 exhibits the same structure as the bark of the beech, remain- 

 ing, however, in this state only from the eighth to the tenth 

 year. About this time there forms in the layer of liber, i. e. 

 only at some places, a delicate layer of tabular cells which 

 agree exactly with that of the periderma. This new layer of 

 periderma is so situnted that a part of the bark substance is 

 completely separated by it, which then gradually dries, and 

 after gradual disunion, actually falls off. These new forma- 

 tions of new layers of periderma are repeated, and thus follows 

 the continual delamination, by which the tree still retains a 

 very even bark. The great scales of bark, which fall oiF, 

 consist, however, of the cellular envelope, and of a portion of 

 the substance of the liber. The scales of the bark in Frunus, 

 Pj/rus, Crataegus, Quercus Robur, Tilia europcjea, &c., are 

 said to originate in the same manner as in the plane tree. 

 Mohl, with other botanists, distinguishes these thick inner 

 layers of the bark of the cork, which are formed in quite a 

 different manner, and calls the inner laye: the rugose bark 

 (rhytidoma, from pung a wrinkle). 



The results of these observations are, that the origin of 

 the scales of the surface of the bark of dicotyledonous plants 

 is not to be sought for in a desiccation of the bark layers, 

 and in a mechanical splitting of them, but that it depends on 



