24 ELEMENTARY BOTANY 
with such stems are described as herbs. When a stem 
becomes woody, it usually forms a shrub or tree. These 
stems generally increase in thickness every year owing to the 
growth of the wood. If a branch of any tree is cut across, it 
is possible to count the rings of wood, and thus to tell the 
age of the tree, for the growth of the wood formed during 
the autumn is closer than that of spring, and as the spring 
growth of one year comes next the autumn growth of the 
previous year, distinct rings are formed. The younger wood 
is on the outside, and as it is permeated with sap, it is known 
as sap-wood, whilst the inner wood igs harder and is called 
heart-wood. This is very well seen in the Laburnum, of which 
a drawing is given in Fig. 28. 
In the Natural History Museum at South Kensington, there 
is a section through the trunk of a tree which was more than 
500 years old when it was cut down. Oaks have been known 
to reach 1,500 years; Yews, 3,200; whilst a tree called Adan- 
sonia, a native of Cape Verd Islands, is said to reach 5,000 years. 
Branching the branching of stems is’ closely connected 
of Stems. with the position of the leaves, for, as we have 
already seen (page 14), buds arise in the leaf-axil. If the 
leaves are opposite, the branches will be opposite ; if alternate, 
then the branches will be alternate. It is very seldom, how- 
ever, that all the axillary buds develop ; as a rule they remain 
undeveloped during the first season, then the following api 
some of them grow into branches. 
It is interesting to notice that the form of trees to a very 
great extent depends on which buds develop. Thus the 
Horse-chestnut is, on the whole, low and wide-spreading, 
whilst the Ash is tall and tapering, yet both have opposite. 
leaves and, therefore, the branches will be opposite ; so that 
at first sight it might have been expected that these two trees 
would have been more or less alike in form. In the Horse- 
chestnut, however, the terminal bud develops into the flower 
and the axillary buds into the branches; as soon as the 
terminal bud has formed a flower, growth in the upward 
direction ceases and must be continued by the axillary buds. 
