55° 



SECONDARY CHANGES. 



very unequal in the different cases. To these differences others are clgsely related, 

 which affect the form of the cork-cells and the cohesion of the layers. 



According to these differences two forms of the superficial formation of cork 

 may be distinguished, though they cannot be separated quite sharply: namely, 

 suherous crusts and suberous integuments. The former consist of numerous layers of 

 soft wide cork-cells, which alternate with thin flat-celled zones, marking the limits of 

 the annual production (p. 114). They constitute superficial masses, attaining a 

 thickness of several millimetres or centimetres, which are soft, and concentrically 

 zoned internally; from their origin onwards they are provided with wing-like 

 projections and deep furrows, because the formation of cork is from the first 

 unequally abundant in alternate longitudinal bands; as growth in thickness goes 

 on they become widely and irregularly cracked. This is especially the case in 

 Quercus Suber and occidentalis, also Q. pseu'dosuber, climbing Aristolochise, e. g. 

 A. cymbifera and A. biloba (cf. Fig. 219), the younger shoots of Acer campestre, 



Liquidambar styraciflora, Ulmus suberosa, 

 Euonymus europseus; species of Banksia 

 and Hakea (Mohl); further, in the above- 

 mentioned cortex of Canelia, the phelloderm is 

 covered externally by thick soft layers of cork, 

 Suberous integuments, the periderm of 

 Mohl, consisting of flat cells only, or, also 

 of thin wide-celled layers alternating with the 

 latter (e. g. Betula, Boswellia, &c., comp. Sect. 

 24), form those smooth coverings of the cortex 

 which are present in the great majority of 

 woody plants. Their bulk in shoots of the 

 same age is very various in different species, 

 according to the amount of the annual new- 

 formation and the extensibility of the walls of the cork-cells. All these con- 

 ditions may remain the same as long as the superficial periderm exists at aU, or 

 they may change at various periods in the age of a shoot. These circumstances 

 determine the extremely various character of the surface in those woody plants which 

 persistently form periderm, to illustrate which some few examples must sufiBce here. 



As already mentioned, many species of Salix (e. g. Salix alba) in their younger years 

 annually form a layer of cork-cells, all successive layers being of the characteristic struc- 

 ture described on p. 549 ; the outermost follow the growth in thickness by their extension, 

 and finally they burst imperceptibly, and peel off, the cork-layer therefore remaining 

 thin and smooth. 



Fagus silvatica forms a highly extensible and firm suberous integument, which, from the 

 first year onwards throughout life, consists of uniform flat cells with brown contents, and 

 receives only a slight successive increment of growth from the meristem. The young 

 stem or branch has therefore a smooth brown surface. The external layers of the 

 suberous integument burst imperceptibly, and wither, while their contents become 

 discoloured. Towards the tenth year of life ^ the process becomes more active, and 

 begins to give the smooth surface that dull whitish colour by which it is permanently 



FIG. 219. — AristoIochiiT biloba ; transverse section of 

 tlie stem, a stronffly developed cork with deep cracks 

 (magniiied about four times). From Schleiden, Grundz. 



' Hartig, Foistlich. Culturpfl. p. 177. 



