AERIAL STEMS 



145 



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47. Aerial Stems. — We may classify aerial steins as to 

 their general character or as to their internal structure. 

 As to internal 

 structure they fall 

 into two great 

 classes based on 

 the arrangements 

 of vascular bun- 

 dles which have 

 just been men- 

 tioned. These will 

 be considered in 

 the section on 

 the structure and 

 growth of stems 

 {Section 50). As 

 to their general 

 character, there 

 are various ways 

 in which stems 

 may be classified ; 

 as to position they may be classified as erect, prostrate, or 

 climbing; as to length of life they may be classified as 

 annual or perennial; as to hardness and strength they may 

 be classified as woody or herbaceous. 



Herbaceous means having to do with herbs. Herbaceous 

 stems are the stems of herbs ; they are generally soft and 

 easily broken. Though they have xylem, they are not 

 woody in the ordinary sense of the term. Herbaceous 

 stems are generally annual, though the plant to which they 

 belong may be a perennial on account of its perennial un- 

 derground parts. Woody stems are perennial. 



Fig. 48. — Cross section of the stem of an annual plant 

 showing the arrangement of the vascular bundles in a 

 cylinder which here appears as a ring. The center 

 is occupied by pith. Outside the vascular bundles is 

 the cortex. 



