THE STEM. 31 



formed over it — thus proving most clearly the order in which 

 the wood and bark are formed. 



57i A remarkable case of the deposition of external layers 

 of dicotyledonous stems is related of the Baobab-tree (Adansonia 

 digitata) of the Cape de Verde Islands. In the year 1400, 

 Grew cut his name on two of these trees, and in 1749 (three 

 hundred and forty-nine years afterward) Adanson examined the 

 same trees and found the names, with more than three hundred 

 layers of wood deposited over them. If we examine a trans- 

 verse section of a trunk of a tree, we observe that the wood 

 near the pith and that near the bark present very different ap- 

 pearances : the latter being white and soft, and more or less 

 juicy, is called the alburnum or sap-wood ; the former, being 

 darker colored and hard, is called the heart-wood. The ves- 

 sels of the alburnum are always filled with sap, and no doubt 

 form the channel through which this fluid ascends. This is 

 shown most conclusively in the process of girdling trees. If the 

 sap-wood is cut completely through all around, the tree dies im- 

 mediately ; but, if a part of this is left, the tree may linger 

 through the summer, and perhaps longer, the continuance of 

 life being in proportion to the amount of sap-wood left uncut. 

 In the young tree all the wood is alburnum, but as it increases 

 in age we may notice the time in which the innermost layer is 

 converted into heart-wood. This change from alburnum to per- 

 fect wood, is no doubt occasioned in a great measure by the de- 

 f Position of foreign matter, which prevents the tissue from any 

 onger performing vital functions, increases its density, and of 

 course renders it more firm -and compact. The time required 

 for the conversion of alburnum into perfect wood, differs con- 

 siderably in different trees, and it is also different in trees of the 

 same species, owing to situation : even on opposite sides of the 

 same tree, the number of layers of alburnum is often different. 

 In trees of the same species, exposed to the same influences, the 

 number of layers of alburnum is remarkably uniform. In some 

 cases there is a striking contrast in the appearance of the albur- 

 num and perfect wood. In the Ebony the alburnum is white, 

 while the perfect wood is nearly black. In the Camb-wood, the 

 alburnum is also white, and the perfect wood a deep red. There 

 seems to be a certain fitness required in the vegetable tissues be- 

 fore they are capable of receiving the coloring matter, for other- 

 wise we should suppose the change would be more gradual : 



57. What remarkable case ? What is alburnum ? What is heart-wood ? 

 How shown that there is a circulation in the alburnum ? How is alburnum 

 ohanged into perfect wood ? What of the time required for the change ? 

 Describe the change in appearance ? 



