ALBURNUM AND DURAMEN. 163 



substance than the autumn wood. The transition from the one to the other in 

 the same annual ring may be gradual or sudden ; and it is to be added that the 

 middle part of an annual ring, between the spring and autumn wood, is often 

 especially rich in libriform fibres. 



On the transverse and longitudinal section of the older stems and branches of 

 trees, the difference between the so-called splint (alburnum) and the heart-wood 

 (duramen) is usually very marked. The alburnum is a pale, more or less thick 

 zone of wood lying under the cortex, and in which two, three, or many annual rings 

 may be recognised; the wood which lies within this zone is generally dark-coloured 

 (red, yellow, brown, or black), and formed of a much harder mass. This is the 

 duramen, alone employed in the arts. Since the thickness of the alburnum remains 

 practically the same, while the duramen increases in thickness year by year, it is 

 obvious that an inner layer of alburnum becomes transformed annually into a new 

 outer layer of duramen. This difference between the alburnum and the duramen is 

 not always evident in the colouring and hardness — in the Fir, for example; and in- 

 deed it is stated of the Box-tree and some kinds of Maple, that this difference does 

 not exist at all, but that the entire wood of an older stem consists of alburnum. The 

 alburnum is the wood in the condition in which it has arisen in a period of vegetation, 

 by the development of the various wood elements from the cambium : it is the normal 

 wood, which comes immediately and alone into consideration for the vital activity of 

 the plant. The water absorbed by the roots ascends in it, and reserve materials are 

 deposited in its medullary rays and other parenchymatous cells. The wood which 

 has passed over into the condition of duramen, on the contrary, takes no more direct 

 part in the vital activity of the plant; it is to be regarded as a mass of tissue passing 

 over into decomposition, and perhaps also destined for the deposition of excretions. 

 The duramen owes its darker hue, as well as its increased hardness and re- 

 sistance to destruction, to the infiltration of dark-coloured substances, often soluble 

 in water as colouring matters (Red-wood, Blue-wood, Yellow-wood) ; or of resinous 

 bodies {Guaiacum, Conifers) ; or occasionally even of silica (Tectona, Iron-wood), and 

 other substances which, like these, in extreme cases not merely impregnate the walls, 

 but partly, or even entirely, fill up the cavities of the fibres and vessels. 



In contrast to the resistent, elastic woody mass, the secondary cortex (secondary 

 phloem) appears on the outside of the cambium-ring, generally as a softer mass of 

 tissue composed chiefly of active living cells ; in this tissue, however, bast-fibres, 

 singly or in groups, produce a conspicuous fibrous structure of great toughness. As 

 already mentioned, the sie\e-tubes are to be regarded as the characteristic main 

 element of the secondary cortex ; in addition to which a larger or smaller quantity of 

 thin-walled, elongated, soft parenchyma is never wanting. It is usual to distinguish 

 both these together as the soft bast, in contradistinction to the proper bast-fibres. 

 The latter are generally very thick-walled, often very long (but occasionally short) 

 tough fibres ; which in some cases are completely wanting in the secondary cortex, 

 and in others are scattered as isolated elements in the soft bast. Frequently," 

 however, these predominate throughout the structure of the secondary cortex, in the 

 form of more or less thick bundles, or even layers. Like the various elements 

 of the wood, those of the secondary cortex may also be present in very different- 

 quantities and variously arranged. 



M 2 



