SECONDARy THICKENING. NORMAL DICOTriEDONS. 513 



every annual ring, independently of its total breadth, from apex to base, so that, 

 for example, the proportion between autumn- and spring-wood in the same three 

 outermost annual rings amounts to i : io-6 at a height of 2'j'^, and to i : 2-5 at a 

 height of I™. The strength and technical usefulness of the wood at different levels 

 on the stem is thus very unequal. According to the statements of technical 

 authorities*, who value the wood of the upper trunk of Dicotyledons and Conifers 

 less than that of the lower, it may be conjectured that similar conditions to those 

 found by Sanio in the Pine are of general occurrence in the stems of trees ; yet 

 these differences in technical usefulness might also have other causes, e.g. differences 

 in the formation of heart-wood, or the causes might be various in the various cases. 

 More extended investigations are still required before any general rules or laws can 

 be laid down. 



Sect. 156. The indistinctness of the limits between the annual rings, which in 

 many woods is typical (comp. p. 503), may also occur in those with a typically 

 sharp demarcation, as an individual phenomenon, especially where the rings are slightly 

 developed ; in cases of eccentric ring-formation this often takes place in such a way 

 that two distinct rings on the stronger side run together into a single one on the 

 weaker. Such an obliteration of the boundaries of rings has often been observed, in 

 the case of Dicotyledonous and Coniferous trees, both in the wood of the stem, and 

 more especially in that of roots '\ Both in the cases of unilateral coalescence of 

 otherwise distinct rings, and in those where the number of the existing complete 

 rings is less than the known age of the wood in years, a distinction must be drawn 

 between the partial or total suppression of the growth in thickness during a period of 

 vegetation, and the suppression of the demarcation of rings where growth takes place. 

 Both cases may occur, the former, for example, has been demonstrated in stunted 

 trees ° ; when a case comes under investigation, it can, as a rule, scarcely be deter- 

 mined subsequently which of the two phenomena has taken place. 



As already indicated, the Araucarias appear, according to the existing data and 

 controversies *, to have a special inclination to the individual differences in question 

 during the occurrence of growth in thickness. Schacht denies the demarcation of the 

 annual rings in A. brasiliensis, even in the face of Goppert's reply. Kraus, on the other 

 hand, describes two pieces of the stem of the same species, one of which had concentric 

 zones, but not a trace of an annual boundary, while the other had eleven annual rings, 

 characterised by sharply defined autumnal and spring-wood. In a portion of the stem of 

 a well-grown specimen of A. excelsa (cultivated in the open ground) I found sixteen 

 rings, which were so sharply marked when seen with the naked eye that one is astonished 

 to hear of its being necessary to seek them with high powers in thin sections. This 

 is explained by the fact that every ring consists principally of thick-walled, tolerably 

 uniform tracheides, and only contains a narrow zone of more thin-walled elements 

 (spring-wood) at the boundary of the next inner ring. These are but little, wider than 

 the thick-walled ones, a sharp boundary between the two being as little observable as a 

 • -distinct flattening of the latter. For these reasons the limit between the rings is by no 



' Compare Nbrdlinger, Techn. Eigensch. d. Hblzer, p. 130. 



' Compare T. Hartig, Forstl. Culturpfl. p. 86.— Von Mohl, Botan. Zeitg. 1862, p. 228, &c.— 

 Kraus, Bau d. Nadelhblzer, /. c. p. 146. — Nbrdlinger, Der Holzring, p. 21. 

 ^ Compare R. Hartig, Botan. Zeilg. 1870, p. 527. 



* On these compare Schacht, Gbppevt, Botan. Zeitg. 1862, and Kraus, I.e. 



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