[Chap. XXIX ROOTS: DEVELOPMENT AND STRUCTURES 



295 



Scuteilum 



Coleoptile 

 Ist leaf 



2nd leaf 



Stem tip 

 3rd leaf 



The primary root continues growth 

 vertically downward for several days 

 at least, and sometimes for weeks or 

 even months after germination. Usually 

 lateral roots grow from it and elongate 

 either horizontally or obliquely down- 

 ward. These are the secondary roots; 

 and when still smaller branches de- 

 velop from the secondary roots, they 

 are called tertiary roots. 



During the germination of seeds like 

 those of corn and wheat, two or more 

 small roots may develop from the base 

 of the first internode of the embr\'0, 

 and still later others from the second 

 and third nodes of the stem. Bv the 

 time the corn plant is two months old 

 there is usually a whorl of roots at each 

 of the eight lower nodes. Each succes- 

 sive whorl appears about one week 

 later than the one below it. These and 

 all other roots that originate from stems 

 or from leaves are adventitious roots. 

 All the roots of a plant — primary, sec- 

 ondary, tertiary, and adventitious — 

 may be called collectively its root sys- 

 tem. The primary root with all its 

 branches may be called the primary 

 root system, and all the adventitious roots of a plant its adventitious 

 root system. 



A mature com root system comprises both of these root systems, but 

 the primary root svstem is limited in growth. Hickory trees, English 

 plantain, and dandelion plants from seed have primary root systems 

 consisting of a large taproot and minor secondaiy roots. All plants 

 artificially propagated by cuttings have adventitious root systems. The 

 root systems of plants that grow from bulbs, tubers, and other under- 

 ground stems likewise are only adventitious root systems after the 

 first season from seed. The above terms are useful in classifying roots 

 according to their origin. 



Another way of classifying roots is based on their forni and appear- 



FiG. 113. Median section through 

 a wheat embryo; c, coleorhiza; p, 

 primary root; r, root cap. After M. 

 A. McCall, Journal of Agricultural 

 Research. 



