172 STEMS 
year, are typical of shrubs and trees, although they occur among 
herbaceous plants, notably in the Ferns, Sedges, and Grasses 
where the underground stem, which is well protected by a 
covering of earth, is able to persist for many years. 
As to position stems are clas- 
sified into aérial, submerged, 
and underground. Submerged 
stems are of least importance, 
being characteristic of plants 
which grow in lakes or slug- 
gish streams, where the plant 
is often supported by the 
buoyant power of the water 
rather than by its stem sys- 
tem. Aérial stems are of 
most importance to us, al- 
though there are some valu- 
able underground stems. 
Fic. 148.— Two stages in the development of a Cabbage plant. A, 
plant at the beginning of the second season’s growth with flowering stem 
pushing out of the head. B, Cabbage plant in flower near the end of the 
second growing season. 4a, scars left by the falling of the leaves of the head. 
Aérial Stems. — Most of our cultivated plants as well as most 
weeds have aérial stems. Since aérial stems keep above ground, 
they are best adapted to expose leaves to the air and sunlight. 
Aérial stems may be erect, prostrate, or climbing. 
