THE STEM 107 
this form of growth are said to be excurrent. In trees with excur- 
rent growth the main trunk extends as a central shaft to the top 
of the tree, while the branches spread from it more or less hori- 
zontally. This results in a conical crown. The opposite form of 
growth is known as deliquescent (Fig. 105). In trees that have 
this form the main trunk is short, while the branches divide into 
smaller and smaller branches, producing a spreading crown. 
Adventitious buds. Most branches originate from buds in the 
axils of the leaves and are called axillary branches. Buds may, 
however, arise from 
the internodes, the 
roots, or even the 
leaves (Figs. 82, 83), 
especially as the re- 
sult of injury. Such 
buds are said to be ad- 
ventitious buds, and 
branches produced by 
them are adventitious 
branches. The growth Fig. 105. Deliquescent crown of a rain tree 
of adventitious buds (Enterolobium saman) 
is made use of in the 
process of pollarding, that is, the cutting back of the tree to the 
trunk to promote a dense growth of branches which arise from the 
tissue produced around the wound. In the case of the willow, 
pollarding produces slender branches, which are used in making 
baskets. In propagation by root cuttings, which is practiced in 
some species, advantage is taken of the fact that some roots pro- 
duce adventitious buds. 

RESPONSE OF STEMS AND LEAVES TO EXTERNAL 
CONDITIONS 
Geotropism. It is a very common observation that stems grow 
upward and roots downward. Even in the case of most pros- 
trate stems the tips tend to grow upward. This tendency of 
stems to grow upward and roots to grow downward is due to 
