166 PHYSIOLOGICAL BOTANY. 



year, and therefore the number of rings on a cross- 

 section does not always indicate the number of years 

 in the age of a tree. But in the higher latitudes a 

 new tube of wood and one of the inner bark is formed 

 yearly. 



That more than one ring of wood may, and sometimes does, form in one 

 season of growth, even in regions of severe frost, has been established by 

 observation. 



418. Sap wood is a name applied to the new wood, 

 and usually includes several of the last formed layers ; 

 it is so called because the fluids in moving upward from 

 the ground pass through its vessels. In most trees it 

 is of a lighter color than the older layers, and on that 

 account was called by early botanists Lignum album, 

 white wood ; now called Alburnum, or white wood. 



419. Heart wood is that part of the trunk or stem 

 near the center or heart, and for that reason called 

 Heart wood. It is usually more dense, and therefore 

 called Duramen, hard wood. In some species it is 

 much darker than the sap wood, hence former botan- 

 ists called it Lignum nigrum, Black wood. In some 

 plants, as the Black Walnut, the Duramen is very 

 dark, while the Heart wood of the Maple is not much 

 darker than the sap wood, though they may grow side 

 by side and draw from the earth the same materials. 

 It would seem, therefore, that chemical changes take 

 place either in the plant's structure or upon the ma- 

 terials taken in to suit the necessities of each case. 



On account of the mode of growth in Dicotyledonous stems, the name 

 Exogens, or outside growers, was formerly applied to plants of this structure. 

 They are characterized by two or more seed leaves in their embryo, and pro- 

 duce netted-veined leaves. See Dicotyledons, pages 163-166. 



Nearly all the trees and shrubs of the temperate 

 zones are Exogens or Dicotyledonous plants, well 



