47(5 SECONDARy CHANGES. 



similar, or equally favourable conditions, the average breadth varies considerably 

 according to the species. Compare, for example, the broad rings of Paulownia and 

 Ailanthus, wiih those of Citrus and Cornus ; Pinus silvestris and Abies pectinata with 

 Taxus, &c. In the stem of the young tree the breadth of the annual rings increases, 

 under otherwise similar conditions, for a number of years, then remaining for a 

 series of years at an average maximum, but decreasing again in advanced age. In 

 the yearly shoots formed on the thickened stem the average maximal breadth of ring 

 is attained in the first year, or in the first few years ^ It is shown even by superficial 

 observation, which may be easily confirmed by more minute research, that the yearly 

 secondary growth in the lateral branches and roots of a tree is less than that in 

 the stem. 



The breadth of each ring is, in the regularly developed s/em, uniform all round, 

 though even in this case it may be unequal in consequence of unequal acceleration of 

 the growth on different sides, the ring thus becoming undulating or eccentric, even to 

 such an extent as to be wholly absent on the deficient side. The rings of one and 

 the same cross-section of the stem often show the most various differences in all 

 these respects, forming, as it were, records of the history of its growth and nutrition ; 

 in certain woody plants, to be mentioned below, such inequalities of growth are 

 typical. 



Unilaterally unequal development of the rings, and consequent eccentric thickening, 

 are the rule for the lateral bra?iches of the stem and of the 7-oois; and in fact in the 

 lateral branches of most Dicotyledonous woody plants, the t/ppt?- side is the favoured 

 one, e. g. Acer pseudoplatanus, Alnus, Carpinus, Cornus, Corylus, Crataegus, Cytisus 

 Laburnum, Euonymus, Gledilschia triacanthos, Fagus, Tilia, Prunus spec, Robinia, 

 &c.^; on the other hand, in the Coniferous woods, and according to Nordlinger in 

 Castanea, the under side is favoured. In small sle?ns also, which for a series of years 

 have grown upright, and increased uniformly in thickness all round, but have then 

 been permanently brought into the inclined position by the pressure of snow, 

 Nordlinger found that the rings formed from the time when the stems became 

 oblique, were eccentric, and that in the Pines, Firs, and Larches the under side, in 

 Oaks and Beeches the upper side is favoured. In the lateral roots of trees at the 

 places where they arise from the stem, the upjjcr side which is continued into the 

 latter is the one favoured ; at a greater distance from the stem the under side usually 

 has the advantage, according to Mohl's opinion^, but this point has not been decided 

 for certain. Centrically developed roots of trees are, however, not actually rare. 



In the case of our forest trees a series of investigations have been instituted on 

 the average amount of the annual secondary growth of the stem in its successive 

 transverse sections from base to apex, which of course always determines the general 

 form of the stem *. In some cases the successive surfaces of the transverse section 



^ Nordlinger, Der Holzring, p. 14. 



- Compare Nordlinger, Holzring, p. 20. — Hofmeister, Allgem. Morphologie, p. 604. 



' Botan. Zeitg. 1862, p. 274. 



* Von Mohl, Botan. Zeitg. 1869. p. i.— Nordlinger, Der Holzring, Stuttg. 1872. — R. Hartig, in 

 Dankelmann's Zeitschr. f. Forst- u. Jagdwesen, Bd. HI; and Botan. Zeitg. 1S70, p. 505. — We may 

 refer to these works for the older, very defective literature, and for many details not strictly be- 

 longing to our present subject. 



