GENERAL STRUCTURE OF STEMS 183 
strengthening tissues assume a tube-like arrangement, which is well 
known to engineers as the arrangement in which the most strength 
with a given amount of material can be secured. The truth of 
this principle is demonstrated in the construction of bicycle 
frames, where much strength with little weight is secured by using 
large tubes instead of solid rods. Again, having the cylindrical 
form, stems can be equally resistant to strains from all directions. 
Stems taper and also decrease in age from the base of the trunk 
to the end of the twigs where 
the stem tissues are in process 
of formation from the apical 
meristems. The apical meri- 
stems are also known as pri- 
mary meristems because they 
form other meristems, notably 
the cambiums. It is on the 
new elongation at the tips of 
the stem, that the leaves appear 
anew each year. The nodes, 
the regions where leaves and 
buds occur, are separated by 
the elongation of the inter- 
nodes, and in this way the 
leaves, which are younger and 
more crowded the nearer the 
tip, are separated and exposed 
to the light. In most annual Fic. 159. Lengthwise section 
stems the nodes are all formed through the stem of a Corn seedling, 
. showing the apical meristems (mm), the 
very early, and elongation nodes (n), and the short internodes (7). 
thereafter consists chiefly in 
the lengthening of the internodes, which thereby separate the 
leaves so that they can unfold and expand to their mature size. 
Thus, as shown in Figure 159, the nodes and internodes of a Corn 
stem are all present in a Corn seedling two or three weeks old. 
In most herbaceous stems, where there is no need of corky bark 
and almost the entire stem is leaf bearing, the stem is active 
throughout in the manufacture of food. But in perennials, such as 
shrubs and trees, photosynthesis is limited to the young branches 
where the leaves are borne and the light is not excluded from the 
green cortex of the branches by a corky covering. In passing 
