THE BRANCHES 
43 
found in most of our forest trees and in many shrubs, 
rhizomes, &c. Here the successive lateral branches in turn 
overtop and supersede the relatively main axis ; the growing 
point, which for the time being is the main one, is pushed 
aside by the growth of the branch, which thus comes into 
line with the axis from which it sprang. The termination 
of the old axis is thus made to look like a branch of the 
straight stem. The second growing point is in turn pushed 
aside and so on, so that the actual straight stem is formed 
of a succession of pieces each added by a separate growing 
point. This is roughly illustrated by the diagram, 
each “] representing one limb of the sympodium, ~J_ 
arising as a branch upon the one below T it. Often -j 
only one limb is formed each year, and during that 
period branches monopodially ; the last lateral bud then 
pushing the terminal one aside in the following year. Excel- 
lent examples of sympodia are : beech (Fagus), Virginian 
creeper and vine (Vitis), the rhizomes of Iris, Juncus, &c., 
the flowering shoot of Geranium pratense , &c. 
Growth in Thickness and Habit of Stem . Most 
erect stems elongate and branch ; they thus increase their 
leaf-surface, and hence also the strain to be borne and the 
demand for water from the roots to supply the transpiration. 
To meet this, they usually grow steadily in thickness, forming 
new wood and bark. A large proportion of the new material 
in erect stems consists of woody fibres or strengthening tissue. 
This thickening process is rare in Monocotyledons and Pteri- 
dophyta, but occurs in a very large proportion of Gymno- 
sperms and Dicotyledons. It is less marked in climbers, 
and rarer in creeping stems, small herbs, &e. (see Chap. III.). 
The living cells of the stem need to respire ; so long as 
the stem is green it has stomata in the epidermis (cf. Leaf, 
below), but as it grows in thickness bark (cork) is formed, 
and this is practically water- and air-tight. To provide for 
respiration special organs ( lenticels ) are formed in the bark ; 
these are portions of bark in Which the cork is loose and 
powdery so that air can pass. They appear upon the surface 
as little eruptive craters full of a brown powder, specially 
well seen in Elder (Sambucus) but easily distinguished upon 
any young twig. 
Storage of reserve-materials is a function very commonly 
