SECONDARY CHANGES OUTSIDE THE ZONE OF THICKENING. 555 



has also been observed in roots (e. g. Sassafras, Vitis, &c.), but always to a small 

 extent, and in a manner not differing from the stems, so far as is known. More 

 accurate investigations only exist as regards the latter. 



In species the first periderm of which arises deep in the cortex (Lonicera, 

 Vitis, Clematis, Cupressineae, &c.) all the later ones assume the same arrangement 

 as the first, and thus cut off in each case an annular layer of cortex, though this 

 is not always quite complete and regular {Ring-bark of Hanstein). In species with 

 a superficial primitive periderm, on the other hand, the successive internal layers 

 arise in such a form, that they abut on the outermost layer of periderm for the 

 time being, and cut off scale-like portions of the cortex (Scale-barK). The single 

 scales have extremely various forms and dimensions even in the same individual. 

 Their formation begins at points which are not morphologically determinate ; the 

 first scale is joined, at or below its edge, by the peridermal layer which cuts off 

 a second scale, next the first, and belonging to the same cortical layer, and the 

 same phenomenon extends round the surface of the stem without any perceptible 

 regularity, cutting off the first cortical layer in scales, and then in the same way 

 attacking a deeper one. The scales which succeed one another at different depths 

 also differ among themselves in form and size ; they do not fit one on another. 



The characteristics of a bark depend, next to the arrangement of its parts as 

 stated above, on the structure of the tissue which becomes dried up, and on that 

 of the periderm, especially of its suberous layers. As regards the former, essential 

 differences are due to the varying hardness and toughness of the desiccated layers 

 of tissue, determined by the occurrence of fibrous and stone-sclerenchyma between 

 the softer tissues, which on drying often become brittle and easily crumble. The 

 thickness of the zone of tissue thrown off each time also has an influence, and is 

 extremely various according to the special case, examples of which will be found 

 in subsequent descriptions. In the cork-layers all the above-mentioned differences 

 of structure occur, and according to their combinations, numerous peculiarities appear 

 in the various species. 



The difference in the cohesion of the membranes of the old cork-cells which 

 undergo desiccation is of primary importance. If they are thin and not very tough, 

 they must become torn, on the one hand by the progressive extension of the 

 periphery of the stem, on the other hand by the shrivelling of the layers of tissue 

 while drying. If a stratum of cork of this nature coats the inner surface of a 

 dry layer of bark, the latter breaks off completely. The well-known scaly bark 

 of the Planes is an exquisite example of this. The stratum of cork bordering 

 a scale is only a few layers thick, and consists in its outer part, adjacent to the 

 scale, of thin-walled, brittle cells, while the more internal ones have thick yellow walls, 

 as mentioned in Sect. 24. The scale is released by the thin-walled zone becoming 

 completely torn through, the thick-walled zone remains till the next following 

 desquamation, as a fairly smooth covering of the living tissue. The surface of 

 the cortex thus remains, on the whole, smooth, with only those flat inequalities which 

 correspond to the outlines of the desquamating portions, and change from year to 

 year. The case is similar in Taxus baccata, in the false quinine bark occurring 

 as China bicolorata, and in the stem of Arbutus Andrachne, A. Unedo, and Salix 

 amygdalina and its allies; Pyrus Malus may also be mentioned here. In the 



