53^ 



SECONDARY CHANGES. 



cortex by the formation of bark show a more vigorous production of bast, corre- 

 sponding to the activity of this process. This phenomenon appears as an instructive 

 individual variation in those stems of Fagus silvatica occasionally occurring which 

 are called Stone-beeches, and are conspicuous from their thick, furrowed bark. The 

 new production of bast no doubt takes place most abundantly in those stems, which, 

 like the vine, annually renew their entire bast-zone, and throw off that of the previous 

 year. 



It is remote from the purpose of the present description to enter into the causal 

 relations of these phenomena. 



Generally valid anatomical characters of the boundary between successive zones, 

 corresponding to the annual rings of the wood, may perhaps still be discovered 

 in the case of the bast, but cannot be determined from the existing data. Even 

 in those cases where regularly alternating concentric zones of non-equivalent tissue 

 appear in the bast, especially fibrous zones alternating with soft bast, their number 

 varies according to the year, the age, and the individual, and a determination of the 

 annual boundaries is therefore usually uncertain. As an example, Hartig's' statement 

 may be cited, that in the case of Willows and Poplars with a smooth cortex, the 

 number of zones of bast-fibres is smaller than the age in years, amounting only to 

 3-4 for every 10-15 years, while, during the development of thick bark-forming 

 cortices, 2-4 fibrous zones arise annually. The species of Acer ^ form in the first 

 years either one, or (at the base of the annual layer) two, successive fibrous zones, but 

 even from the sixth year onwards the relation changes in such a manner that often 

 only 20-25 fibrous zones correspond to 100 years. The numbers are more regular 

 in Tilia, where, according to Hartig ", four fibrous zones appear at the base and one 

 at the apex of the shoot in its first year, to which two or three are added in the second 

 year, and in each succeeding year, on the average, two ; this is also the case in Pyrus 

 communis, which, according to Mohl *, forms one fibrous zone annnally. 



In those woody plants also, which renew their cortex every year, and in which 

 the limit of the annual increment of growth is sharply defined by the layer of periderm 

 formed at its outer side (Sect. 177), similar difi'erences to those just mentioned appear. 

 Lonicera Caprifolium and its allies annually form one zone of fibres and one of soft 

 bast; Clematis Vitalba usually two of each^; Vitis vinifera generally forms two 

 fibrous zones alternating with soft bast, at the close of the first period of vegetation, 

 while in later years 3-5 are generally formed annually ". 



W.f. p. 444. W.C. p. 547. 



= I.e. p. 560. * Botan. Zeitg. 1855, P- 880. 



" Hanstein, Baumrinde, pp. 72, 77. 



" Hanstein, I.e. p. 61. — Von Mohl, I.e. p. 879. 



