72 



BOTANY OF CROP PLANTS 



leaf which is wrapped about the stem) of the first few leaves, 

 or spring freely from underground stems. They may also 

 arise from joints above the ground hne, as in corn. If such 

 aerial roots reach the ground they may serve as supporting 

 or "prop" roots (Fig. 56). 



Stems. — General Characteristics. — The stems of grasses 

 are called culms. They are cylindrical (rarely flattened), 



and divided into sections 



Stem- 

 jheatb 



drowinO 

 zones 



°& 

 internode^ 



node 



(internodes) (Fig. 25) which 

 are usually hollow, but some- 

 times filled with pith, as in 

 corn. When young, the in- 

 ternodes are solid, but, as 

 the stem enlarges, the central 

 portion is ruptured and a 

 hollow is formed. The nodes 

 (Fig. 25), the enlarged joints 

 between the internodes, are 

 soHd. Enlargement of the 

 nodes is due partly to a 

 thickening of the leaf base at 

 each node (Fig. 26) and 

 partly to enlargement of the 

 stem itself. In most grasses, 

 the part of the culm within 

 the sheath remains soft and 

 continues to grow or retain 

 the power of growth after the portion not in the sheath has 

 ceased growth, or lost the ability to grow. The youngest 

 part of each internode is at its base, surrounded by the basal 

 swelling of the leaf sheath (Fig. 26). Each internode has its 

 own growing zone. 

 Lodging. — It is customary to speak of a grass as "lodged" 



Fig. 26. — Barley. A, portion of 

 leaf at juncture of leaf and blade; 

 B, stem cut in median lengthwise 

 section. x 2}i. 



