10 



GRASSES OF IOWA. 



and alternate with the soft tissue (phloem). The woody ele- 

 ments consist of scalariform ducts, spiral tracheids and an 

 annular duct. 



The primary or seminal root is of short duration, and is soon 

 replaced by the secondary, or as they are commonly called, 

 "brace roots. " These roots show conspicuous large bundles 



Fig. 5. Root of Maize, a, root-cap of loose cells; 8, growing point; e. epidermis; i', thick 

 walls; m, g, f, plerome, central cylinder; y, a ves&el; /, wood; m, pith; x, r, cortex 

 (Sachs » 



in which occur spiral tracheids, annular and scalariform 

 ducts. The secondary roots arise from the nodes, and ordi- 

 narily are aerial, though some are formed soon after the seed 

 germinates. These appear as swellings, the continued growth 

 within causing the stem to become ruptured, and allowing the 



