SECONDARY CHANGES OUTSIDE THE ZONE OF THICKENING. 551 



characterised throughout Ufe (if rare exceptional cases, in which bark is formed later, be 

 left out of consideration). 



Similar conditions occur in other woody plants with a permanently smooth suberous 

 integument, e. g. species of Carpinus and Planera. Others, as for example the species of 

 Prunus, likewise form, so far as is known, only uniform, flat-celled integuments of cork, 

 but the latter are thicker, and extremely tough, and long resist the action of the 

 weather. In consequence of the latter properties the outer layers, when finally burst, 

 adhere to the surface of the cortex as tough membranous flaps. 



The shoots of Corylus Avellana form in the first year a suberous integument consisting 

 of wide thin-walled cells. The latter soon bursts, and forms the yellowish gray, easily- 

 peeled coating of the one-year-old shoots. Later on, firm, flat-celled, and soft wide- 

 celled layers of cork (one or two strata of cells thick) are alternately formed ; the latter 

 burst readily, the firm ones peel off from them in shreds, which remain adhering to the 

 shoots when the latter are several years old. 



Betula alba forms from the first year onwards a firm suberous integument consisting of 

 uniform flat cells with brown contents. The shoots therefore possess in the first instance 

 a smooth brown surface. Later on, beginning at about the fifth year, wide-celled, thin- 

 walled layers alternate with the flat-celled ones, the former being at first simple, but 

 afterwards consisting of several or many strata of cells. In later years the brown mass 

 of contents is absent, even in the newly-formed flat-celled layers, the entire suberous 

 integument becoming colourless. The tearing of the thin-walled layers results in the 

 peeling oflT of the corky integument. 



The remarkably tough, leathery, thick suberous integuments of Bosivellia papyrifera 

 split into sheets owing to the tearing of the thin, brittle, silicified layers, which were 

 described above in Sect. 24. 



Sect. 176. A number of stems and branches of Dicotyledons, and almost all 

 roots of Dicotyledons and Conifers, undergo a profound anatomical change on the 

 formation of periderm, inasmuch as the latter takes place in the interior, at a 

 considerable distance from the surface, and all parts lying outside the periderm, being 

 cut oif by the corky layer from the access of the sap, die away. The masses of tissue 

 thus cut oif are termed the hark (Mohl, /. c). 



The internal periderms arise round the whole member, by means of the pro- 

 cess of development generally characteristic of peridermal formations, in a surface 

 which is everywhere approximately equidistant from the centre, and which follows 

 the periphery of the bast. Its transverse section, corresponding to that of the 

 bast, is either circular, or indented opposite the medullary rays. According as the 

 general surface of the member is similar or dissimilar to that of the bast, the 

 peridermal layer therefore lies either at a constantly equal, or at a variously unequal 

 distance from the former. Thus, for example, it surrounds the circular transverse 

 section of the bast in Thuja and Juniperus as a ring, lying at a great distance from 

 the surface opposite the corners of the branches, and only separated from it by a 

 narrow zone of parenchyma between them. Between the corners or projections of 

 the branches of Casuarina, it may even lie immediately below the epidermis. 



With reference to its special position, it will be best to distinguish between stems, 

 or their branches, and roots. 



I. It has already been stated that in many stems of Leguminosse the initial 

 layer for the formation of periderm is the second or third layer of cortical cells from 

 the outside. These cases effect the transition to those, which have to be considered 

 here, with a more deeply situated initial layer. 



