THE STEM. 41 



or less juicy, and is called the alburnum or sap-wood ; the for- 

 mer, being darker colored and iiard, is called the hcart.xcood. 

 The vessels ol' the alburnum are always tilled with sap, and 

 no doubt tbrni the channel through which this fluid ascends. 

 This is shown most conclusively in the process ot' girdling 

 trees. It' the sap wood is cut completely through all round, 

 the tree dies immediately ; but, it' a part of this is left, the 

 tree may linger tiirough the summer, and perhaps longer, the 

 continuance of lile being in proportion to the amount of sap 

 wood left uncut. In the voun«r 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 perfect wood, is no doubt occasion- 

 ed in a great measure, by the deposition of foreign matter, 

 which prevents- the tissue from any longer performing vital 

 functions, increases its density, and of course it becomes 

 more firm and compact. The time required for the conver- 

 sion of alburnum into perfect wood, ditTers considerably in 

 ditTerent trees, and it is also diflerent 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 difierent. 

 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 alburnum and perfect wood. In the Ebony, the albur- 

 num is white, while the perfect wood is nearly black. In 

 the Cam!). wood, the alburnum is also white, and the per- 

 fect wood a deep red. There seems to be a certain fitness 

 required in the vegetable tissues before they are capable of 

 receiving the coloring matter, for otherwise we should sup- 

 pose the change would be more gradual ; but the line of de- 

 markation is ol'ten perfect; the black external layer of the 

 perfect wood being surrounded by a perfectly white layer of 

 alburnum, thus showing that the transition is performed at 

 once, and not gradually as is generally supposed. The for- 

 mation of wood will more properly bo noticed after the func- 

 tions of leaves have been jjiven. 



48. The pith is the central portion of the stem commenc- 

 ing at its base and extending through it and through every 

 branch, terminating in the buds. It is composed of loose cel- 

 lular substance, varying considerably in size, form and ap- 

 pearance in different plants. In some plants it forms a large 

 portion of the stem as in the Elder ; in others but a small 

 part, and in trees, becomes compressed into a mere line, as 

 in the Oak. It never increases in quantity in the same part 



4* 



