19 A TEXT-BOOK OF BOTANY 
and work of roots. One of the most obvious contrasts 
with the stem in external appearance is that roots bear no 
leaves or scales, and are not made up of nodes and inter- 
nodes. 
The root that comes from the seed, including all of its 
subsequent branches, is the primary root. In some cases 
the primary root develops a single prominent vertically 
descending axis, called the tap-root, which gives off small 
branches, as in the dandelion (Fig. 68, A); in other cases 
the primary root breaks up at once into a cluster of branches, 
as in many grasses (Tig. 68, B). In many cases the tap- 
root becomes conspicuously thickened for food storage, as 
illustrated by such common 
vegetables as radish (Fig. 69, 
A), turnip, and parsnip. In 
some cases where there is no 
Fic. 69.—Fleshy roots: A, radish with fleshy tap-root; B, dahlia with cluster of 
fleshy roots 
tap-root, the branches become thickened, forming such 
clusters of thickened roots as those of the dahlia (Fig. 69, 
B) and of the sweet potato. Roots that arise from the 
stem or the leaves are secondary roots. For example, a 
subterranean stem or a creeping stem strikes root from the 
