156 THE STRUCTURE OF ECONOMIC PLANTS 



structures extends the slit in the pericarp until the coleoptile 

 is exposed. After the coleorhiza has elongated about a millimeter, 

 the enclosed primary root digests and pushes its way through it. 

 Shortly thereafter, the first pair of seminal roots emerges from 

 the sides of the short hypocotyl, near the cotyledonary node; 

 and these three roots grow with about equal rapidity until, at 

 the end of two or three days, there is little difference in their 

 length or thickness. (Figs. 58, 6i.) At about this time, the second 

 pair of seminal roots emerges above the first pair; and, in some 

 cases, a sixth rootlet may develop behind the epiblast. 



McCall (9) reports that he and Taylor observed that in most 

 cases only three roots developed; but in a fairly large number of 

 cases, four developed — the primary root, the first pair, and one 

 of the second pair of seminal roots. In a smaller number, five 

 roots developed; and, "as a rule, only relatively few seedlings 

 of most varieties of wheat have the full potential complement 

 of six seminal roots." 



During the development of the seminal root system, the cole- 

 optile elongates and the enclosed foliage leaves enlarge, the 

 former protecting the leaf primordia and stem apex as they grow 

 through the soil. The degree of emergence of the coleoptile is 

 conditioned by depth of planting and environmental factors; and, 

 after the tip of the coleoptile is above the soil surface, the first 

 and second foliage leaves push through the small pore near its 

 apex. Where wheat is sown at a depth of i| to x^ inches, there 

 is little elongation of the first internode. After several days, 

 the second internode elongates until the third node, from which 

 the first foliage leaf is diverged, is raised to the level of the soil 

 surface or just below it. The bud in the axil of the coleoptile 

 develops, and adventitious roots may arise from the internode 

 above. At this time, elongation of the second internode ceases, 

 and the roots which constitute the first whorl of the adventitious 

 root system are formed. 



Under average conditions the lower internodes, above the first 

 whorl of adventitious roots, do not elongate much; and the pro- 

 duction of roots at the nodes, together with the development of 

 the axillary buds, results in the formation of several secondary 

 axes or tillers and a fibrous secondary root system. 



The Primary Root. — The primary root has an exarch, radial 

 protostele with from seven to eleven protoxylem points. At a 



