SECONDARY CHANGES OUTSIDE THE ZONE OF THICKENING. 559 



twenty-fifth to thirty-fifth year of life ; in the Alders in the fifteenth to twentieth, the 

 Limes in the tenth to twelfth, in Salix amygdalina in the eighth to tenth ; in other 

 Willows, with relatively abundant bark, earlier. In the Birch (B. ajba), the formation 

 of bark begins on the stems, from the fifth or sixth year of life onwards, starting at 

 the base, and gradually advancing upwards, seldom to a greater height than four 

 metres. Populus tremula keeps its smooth superficial periderm for many years ; 

 in P. nigra and pyramidalis it is soon thrown off by the formation of bark. 



According to Mohl' the stem of Pinus sylvestris and nigricans begins the 

 formation of its thick scale-bark in the eighth or tenth year. 



3. A relatively small number of woody plants, mentioned above at p. 552, form 

 their first periderm deep down in the cortex, and thus cast off the external layers of 

 the latter at once. In the known cases this always happens during the first period 

 of vegetation of the shoot, or at the limit between this and the second. In the 

 further behaviour the following different phenomena then appear. 



(a) After the formation of the first periderm the shoot remains covered by it; 

 it follows the growth in thickness throughout life, or at least for years, and a renewed 

 formation of internal periderm and bark only appears at a late period. In the stem 

 of Cobaea scandens, after the external cortex has been cast off in the first year, and the 

 bast-layer coated by an irregular periderm, no further internal formation of periderm 

 has been observed. The latter occurs in Ribes (according to Hanstein, I.e.); also 

 in Punica Granatum, where the one-year-old branch, after it has become coated with 

 periderm, and has cast off the external cortex, as mentioned above, may grow for 

 10-20 years in thickness, before a new formation of bark (no doubt in the annular 

 form) once more throws off a narrow cortical zone. Among Coniferous trees may 

 here be mentioned Pinus sylvestris and nigricans, in which the casting-off of bark 

 begins on the stem at the eighth to the tenth year ; Larix, with a formation of scaly 

 bark beginning approximately in the eighteenth year, and afterwards frequently 

 repeated; and Abies excelsa, where this process begins about the twentieth year^. 



{d) The first annular internal formation of periderm is followed by that of 

 successive new zones, of similar arrangement, at short intervals, which have not been 

 more accurately determined, and without exact, or at any rate without exactly 

 investigated, relations to the annual secondary growth of the bast. So in the majority 

 of the above-mentioned woody plants with annular bark, e. g. Melaleuca, Callistemon, 

 Cupressineae, &c., the cortex of which becomes covered within a few years by several 

 layers of bark. 



(c) In every period of vegetation succeeding the first a new zone of bast is 

 produced, and at the end of the period the whole zone belonging to the previous 

 year is cast off by formation of periderm : Vitis, Clematis, Atragene and Caprifolium. 



4. So far as is known, the woody plants investigated either show the same 

 behaviour in all individuals, or hereditary individual differences. Examples of the 

 latter have been mentioned above in the case of the Stone Beech, and the Cork 

 Oaks which form bark; the variety of Ulmus effusa, known as the Cork Elm, 

 with large wing-like outgrowths of cork, on the surface of the first periderm of 

 young shoots, which is cast off about in the sixth year, may also be mentioned here. 



» Botan. Zeitg. 1859, P- 338- ^ Von Mohl, /.c 



