SECONDARY THICKENING. NORMAL DICOTYLEDONS. ^\t 



As has often been indicated in the preceding pages, the transformation or 

 degradation of the alburnum into duramen takes place in different species of trees 

 at a diiferent average age, and in some gradually, in others suddenly. In the 

 stem of Fraxinus excelsior when forty years old, and in that of the Birch when thirty- 

 five years old, abundance of starch was found by Gris in all the annual layers ; the 

 former species is, according to Nordlinger, characterised by a very broad alburnum", 

 while Betula alba is an ' alburnum tree.' In a stem of Fagus sylvatica ninety-five 

 years old, investigated by Gris, there was abundant starch in the fifteen outermost 

 annual rings, then a gradual decrease in its amount from these up to the thirty-fifth 

 ring, while further inside it was wholly absent. According to Nordlinger, Quercus 

 pedunculata has 8-13 annual layers of alburnum; in an oak-stem fifty-eight years 

 old, Gris found that the cells ceased to contain starch somewhat suddenly at the 

 sixteenth ring, and in one ninety-eight years old, at the twenty-first ring. Robinia 

 Pseudacacia has 3-5 rings of alburnum, which are sharply marked off from the dark 

 heart-wood, both by their containing starch, and by their light colour : Castanea 

 vesca behaves in a similar way (Gris and Nordlinger). According to statements 

 based on those coarser dilFerences which are of technical importance, the diversity 

 in this respect is very great. 



It is equally evident from the facts already mentioned, that in the same species 

 of tree, and even in the same individual, stem, or branch, numerous individual 

 variations occur, within definite specific limits, according to the age and vigour 

 of development, especially of wood-formation, and at different levels of the stem. 

 This is the case whether the relative thickness of alburnum and duramen be deter- 

 mined according to the number of annual rings, or according to absolute measure. 

 Even on different sides of the same cross-section the number of annual rings 

 showing the properties of alburnum is often different ^. 



What has been hitherto said applies to wood enclosed in the uninjured stem. It is 

 well known that changes not unfrequently occur in the wood in consequence of injuries, 

 and that the extent of these changes, which are similar to those characterising the 

 formation of heart-wood, shows a definite relation to the position and extent of the 

 wound. The processes of decomposition to which they owe their origin may often be 

 different to those which accompany the formation of duramen ''. On the other hand, the 

 phenomena occurring in both cases are often so similar — as for example the conversion 

 into resin in the Coniferse, and the formation of black, hard wood in the Ebenaceae — that 

 we may suppose the same process, which usually comes on more slowly, to be accelerated 

 or excited by injury. 



e. Individual and local deoiations. 



Sect. igg. Within the limits of the typical characteristics, which have been 

 explained in the preceding pages, the structure of the annual ring in the same 

 species shows definite variations, according to differences, however produced, in the 

 vigour of its development ; the single feebly developed ring, or even portion of a 



' Compare especially the detailed statements respecting the Oak in Duhamel, Physique des 

 Arbres, I. p. 46, &c. 



^ Compare e. g. Nordlinger, /. c. p. 37. 



