THE STEM 123 
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 
: Fig. 142.— Section 
room for themselves, bring about an in- of tree trunk showing 
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 init. 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 
Fics. 143-144. —Dia- 
gramsof tree trunks, show- With the age of the knot as deduced 
ing knots of different ages? from its own rings? As the tree may 
148, from tree grown in 
the open; 144, from tree continue to live and grow indefinitely 
grown ina dense forest. tter 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 
