THE STEM OF VASCULAR PLANTS. 155 



in the second year, a second layer, placed immediately 

 on the outside of the first, surrounds it again on all 

 sides ; and, in like manner, year after year ; the only 

 essential difference which is observed between the fixst 

 and following layers, is that the first presents, even at 

 an advanced age, tracheae in a state capable of unrol- 

 ling ; and that in the following, even in a young state, 

 there are only found striped or dotted vessels. Each 

 year the vessels already formed acquire more hardness 

 and firmness, because the juices which traverse them are 

 continually depositing small particles in them. It hap- 

 pens that after an indefinite number of years the layers 

 are unable to take any farther consistence ; those which 

 are still sufficiently young to acquire new particles, form 

 the Alburnum; those which cannot acquire more, the 

 Wood, We know, from this plain and simple expo- 

 sition of the fact, that the alburnum is less tenacious, 

 firm, and compact, than the wood; we know that the 

 different layers of the alburnum can present different 

 degrees of firmness according to their age ; whilst 

 those of the wood, having arrived at their maximmn 

 of hardening, ought to present a more homogeneous 

 mass, although all of different ages. 



In some trees, and especially in those which are not 

 very hard, the line of demarcation of the wood and 

 alburnum is hardly perceptible ; we see this in the Pop- 

 lar, the Willow, the Chestnut, the Bomhax, &c. : on the 

 contrary, in hard woods, this line is readily distinguished 

 by the hardness and colour of the organs ; thus, in the 

 Ebony, the wood is, as every one knows, perfectly black, 

 whilst the alburnum is white (PL 5, fig. 9) ; in Cercis 

 Siliquastrum the wood is yellow and the alburnum 

 white ; in PhiUyrea the wood is brownish yellow, the 

 alburnum white ; but in this last species the perfect 

 wood is only found in very old trees ; and I have 



M '^. 



