12 INTRODUCTION TO BOTANY 
stem will be discussed in later chapters. By means of their 
upright position, stems may support the branches and leaves 
in such a way that they are upheld in the air. Support is evi- 
dently one of the conspicuous functions served by plant stems. 
Many plants live for only one year (wnuals) or two years 
(biennials) ; in such plants large and strong stems are not 
often found. Other kinds of plants may live for two or more 
years (perennials). Woody perennials may live for hundreds 
of years, and their stems usually 
increase in thickness from year 
to year, until, in the case of trees, 
stems are sometimes developed 
which are several feet in thick- 
ness, much over a hundred feet 
in height, and of such strength 
that very heavy tops are upheld 
throughout the severest storms. 
The increase in thickness is shown 
by the annul rinys of wood (fig. 8). 
Often the age of a woody stem 
Fia, 8. 
One quarter of a cross 
section of a stick of oak wood 
m, medullary rays, running from 
bark to pith; ., annual rings; b, 
boundaries between rings, porous 
from presence of many ducts; @, in- 
terior fibrous layers of dead bark ; 
pl, hard plates of dead bark, split- 
ting away from each other but at- 
may be indicated approximately 
by its annual rings, but that these 
are not always truly annual rings 
is shown by the fact that when 
tached to bark beneath. Reduced fi é 
there are intermittent favorable 
and unfavorable growing periods within the same season, 
more than one rmg may be formed within one year. Cases 
are known of young trees that have almost twice as many 
rings as the number of years of their age. 
10. Stems as passageways for liquids. When the stem of 
an actively growing plant is cut, water sometimes exudes 
upon the cut surface of the stump. The same outpouring of 
water is seen when a leaf of a corn plant is peeled away from 
the stalk. When we wish to keep cut flowers in a fresh con- 
dition, we place their stems in water. If we should place a 
fresh leaf of celery in a solution of one of the aniline dyes 
