THE STEM 



123 



FIG. 142. Section 

 of tree trunk showing 

 knot. 



144 



increase of volume by the absorption of liquids is termed 

 imbibition. Care must be taken not to confound imbibi- 

 tion with capillarity. (Exp. 53.) When liquids are carried 

 into a body by capillary attraction, they 

 merely fill up vacant spaces already exist- 

 ing between small particles of the substance, 

 and therefore do not cause any swelling or 

 increase in size. When imbibition takes 

 place, the molecules, or chemical units of the 

 liquid, force their way between those of the 

 imbibing substance, and thus, in making 

 room for themselves, bring about an in- 

 crease in volume of the imbibing body. 

 To this cause is due the alternate swelling and shrinking of 

 timber in wet and dry weather. 



137. Knots. Look for a billet with a knot in it. Notice 

 how the rings of growth are disturbed 

 and displaced in its neighborhood. If 

 the knot is a large one, it will itself 

 have rings of growth. Count them, and 

 tell what its age was when it ceased to 

 grow. Notice where it originates. 

 Count the rings from its point of origin 

 to the center of the stem. How old was 

 the tree when the knot began to form? 

 Count the rings from the origin of the 

 knot to the circumference of the stem ; 

 how many years has the tree lived since 

 the knot was formed ? Does this agree 

 with the age of the knot as deduced 

 from its own rings? As the tree may 

 continue to live and grow indefinitely 

 after the bough which formed the knot 

 died or was cut away, there will probably be no corre- 

 spondence between the two sets of rings, especially in the 

 case of old knots that have been covered up and embedded in 



FIGS. 143-144. Dia- 

 grams of tree trunks, show- 

 ing knots of different ages : 

 143, from tree grown in 

 the open ; 144, from tree 

 grown in a dense forest. 



