130 GROWTH. 



yields to the extension of the stem, and exfoliates but 

 slowly. 



205. In regard to the separation of plates or scales 

 from the bark of plants, Mohl observes, (speaking of 

 the Genus Prunus more especially,) that the external 

 coat of the bark remains smooth for some time in con- 

 sequence of a formation of new layers upon its internal 

 surface, and a gentle exfoliation at its external one, and 

 acquires at the same time a sensible thickness (Prunus 

 domestica), but in the end tears off as in the Birch, and 

 the surface of the stem becomes rough and scaly ; but 

 the scales are not as in the Birch, the result of a new 

 development of parenchymatous matter external to the 

 stratum parenchymatosum, but have their origin at an 

 expense of the parenchymatous layer itself, and librous 

 one as well ; but Physiologists have been wrong in sup- 

 posing, that the scales are the result of a mere desicca- 

 tion and simple exfoliation of the bark ; the layers of 

 the bark forming the scales are separated by thin layers 

 of new tissue developed in the third or fourth stratum of 

 the bark, before they become completely detached ; the 

 separation of each portion being thus as it were pre- 

 pared. In Prunus, a series of superimposed scales are 

 found, which separate at first by their edges, but remain- 

 ing for some time fixed upon the stem, whilst in the 

 Plane-tree the scales fall as they form. 



206. We shall now turn to the Endogenous stem, and 

 we shall find that on examining one, and comparing it 

 with that of an Exogenous plant great differences in 

 appearance present themselves. 



207. In an Endogenous stem no true central cellular 

 system or pith is to be seen, the nearest approach is to 

 be met with in Palms, in which the cells of the general 

 cellular tissue continue pretty large in the centre of the 

 plant, and are often filled with amylaceous matter ; but 

 even through this part bundles of woody fibre and vas- 

 cular tissue run along, and no separation of the cellular 

 mass from the rest of the stem is to be observed. 



208. The woody matter is arranged as bundles of 



