GROWTH BY THE STEM. 27 



sisting of -woody matter mixed with cellular. The 

 external is the rind or cortical integument, the inter- 

 nal is the liber. These two parts grow independently 

 of each other, by their inner faces ; the rind belong- 

 ing exclusively, to the horizontal system, the liber 

 composed of the perpendicular and horizontal systems 

 intermixed. 



49. In all Exogenous plants whose stems acquire 

 an age beyond that of a very few years, the wood is 

 distinguishable into two parts, heart-wood, and sap- 

 wood or alburnum. The former is more or less cen- 

 tral, and coloured brown or some dark tint ; the lat- 

 ter is external, pale yellow, and much softer. Heart- 

 wood was originally alburnum, and altered its nature 

 with age, in consequence of the solid matter with 

 which all its tubes and vessels were choked up ; al- 

 burnum is the youngest wood, with all its communi- 

 cations free and open, no solid matter having had 

 time to accumulate within them. The reason why 

 solid matter collects in the tubes of wood, so as gra- 

 dually to choke them up, is this : the wood is the 

 channel through which all the fluid matter of a plant, 

 whether crude or digested, passes, in its way upwards 

 to the leaves, or in its horizontal direction from the 

 bark to the central parts of the stem. "When sap 

 leaves the earth and passes into the stem, it ascends 

 by the woody matter of the finest fibres of the root ; 

 having left them, it flows into the new wood from 

 which those fibres emanated, and passes along this 

 until it reaches the leaves : on its return from them 



