33 PRACTICAL FORESTRY. 



for posts, rails, shingles, or other articles used in outside 

 work, but it is sometimes used for inside finishing of 

 buildings and furniture, giving greater variety. The 

 young or outside layers of wood are the tougliest and 

 most flexible, as the filling up or thickening of the cell 

 wall of the heart-wood makes the timber more firm and 

 rigid, and doubtless more durable, but at the same time 

 its elasticity and toughness is diminished. 



That the inside or heart-wood is dead, and only serves 

 to strengthen the tree mechanically, is shown in the fact 

 that it may be removed entirely by decay, and still the 

 tree grow on vigorously for centuries. This leads me to 

 the subject of 



THE MOVEMENT OF SAP IN TREES. 



All plants obtain their nourishment in a liquid or 

 gaseous form, by imbibition through the cells of the 

 younger roots or their fibrils. The fluids and gases thus 

 absorbed, i:)robably mingling with other previously as- 

 similated matter, is carrried upward from cell to cell 

 through the alburnum or sap-wood until it reaches the 

 buds, leaves, and smaller twigs, where it is exposed to 

 the air and light, and converted into organizable matter. 

 In this condition a part goes to aid in the prolongation 

 of the branches, enlargement of the leaves, and formation 

 of the buds, flowers, and fruit, and other portions are 

 gradually spread over the entire surface of the wood, ex- 

 tending downward to the extremities of the roots. We 

 often speak of the downward flow of sap, and even of its 

 circulation, but its movement in trees in no way corre- 

 sponds with the circulation of blood in animals, neither 

 does it follow any well-defined channels, for it will, when 

 obstructed, move laterally as well as lengthwise or with 

 the grain of the wood. 



The old idea that the sap of trees descended into the 

 roots in the fall and remaining there through the winter, 



