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 weU protected by a 

 covering of earth, is able to persist for many years. 



As to position stems are clas- 

 sified into aerial, 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. Aerial stems are of 

 most importance to us, al- 

 though there are some valu- 

 able underground stems. 



Fig. 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, a, scars left by the falling of the leaves of the head. 



Aerial Stems. — Most of our cultivated plants as well as most 

 weeds have aerial stems. Since aerial stems keep above ground, 

 they are best adapted to expose leaves to the air and sunlight. 

 Aerial stems may be erect, prostrate, or climbing. 



