GROWTH OF TREES. 203 



do not extend in length excepting at their extremities, it 

 was not meant that that should apply to the shoots of 

 the year. All the parts of a plant continue to extend 

 in length when they are in a state of cambium ; and 

 when they pass from that state, they either harden and 

 wither, or are consolidated into wood and bark. The 

 complete change for the pulpy or viscid cambium (for 

 it is sometimes the one and sometimes the other) to 

 solid wood and bark, does not wholly take place in one 

 season, and generally speaking it is longest in taking 

 place in the most durable timber. In the young and 

 unconsolidated state, the wood is alburnum or sapwood, 

 and in the bark it is liber ; and it does not appear that 

 the change from alburnum to hearty wood, is so gra- 

 dual as one would be led to suppose. In the pine, the 

 oak, the laburnam, and many other trees, alburnum 

 may be distinctly traced to a separate ring, there being 

 a greater or smaller number of layers of sapwood ac- 

 cording to the species of tree and the time it has taken 

 to grow. There is no doubt that the consolidation of it 

 is promoted by cold, and by slowness of growth. There 

 is less sapwood upon the pine that grows on the moun- 

 tains of Scandinavia, than in that which grows upon 

 the mountains of Scotland ; and there is less upon the 

 north side of a pine than the south, if the cold winds 

 come to it from the north. Indeed when pines are in 

 too warm a situation, they are altogether little better 

 than sapwood ; while stunted pines and small gnarled 

 oaks in cold and exposed situations, are hearty wood 

 almost to the very bark, and the annual additions are 

 so small that they cannot be seen without a micro- 

 scope. In many of the deciduous timber trees, the 



