144 



THE STRUCTURE OF ECONOMIC PLANTS 



nodes, although five or seven are not uncommon. The basal 

 internode remains short, while the second one elongates so that 



the first adventitious roots of the 

 secondary root system are about an 

 inch below the surface of the soil. 

 The internodes above the second 

 are successively longer, the upper- 

 most one which bears the terminal 

 spike being the longest. The di- 

 ameter of the stem is influenced by 

 various factors; but it increases up 

 to the fifth internode, while the 

 sixth is more slender than the rest. 

 Secondary shoots or tillers arise 

 from axillary buds at the basal 

 nodes of the main axis; and from 

 these lateral culms, still others 

 may develop. In this manner, the 

 process of tillering results in the 

 formation of several axes of the 

 second and third order until many 

 branches have been produced. The 

 behavior of the axillary buds in 

 forming tillers depends in part on 

 the depth of planting of the grain. 

 When the depth is less than 2. 

 inches, the bud in the axil of the 

 first foliage leaf develops immedi- 

 ately; at about 2. inches, the third 

 axillary bud is usually the first to 

 grow vigorously; and where plant- 

 ing is 3 or more inches deep, the 

 first three buds of the main axis 

 remain dormant or grow weakly, 

 while the fourth lateral bud pro- 

 duces a strong stem. 



The Leaves. — The alternate, 

 Fig. 59. The root system at the time two-ranked leaves are of Several 



of blossoming. (From Weaver R../D.- j^jj^jg. ^^^ ^^^ seedling leaveS 

 velofment of Field Crops. McGraw-Hill ^^ -inz-ni 



Book Co.) (scutellum and coleoptile}; {2.) the 



