636 



THE STRUCTURE OF ECONOMIC PLANTS 



stl 



CO 



-en 



The tip of the primary root emerges from the basal end of the 

 fruit coat and grows rapidly downward through the soil. This 

 is followed by an elongation of the hypocotyl which raises the 

 cotyledons and the partially enveloping remains of the akene. In 

 some instances, the cotyledons are freed from the combined fruit 

 and seed coats as they emerge from the soil. At the time of 



emergence, the cotyledons 

 are in a horizontal posi- 

 tion; but as growth pro- 

 ceeds, they are reoriented 

 until they are nearly verti- 

 cal. (Fig. 331.) As the 

 two blades expand later- 

 --pcl ^22y^ j-j^g minute epicotyl 



is exposed; and, within a 

 few days, the first foliage 

 leaf appears. (Fig. 332..) 

 The formation of lateral 

 roots is initiated early; 

 and in about six days, 

 secondary roots appear on 

 the first inch and a half 

 of the primary root . They 

 are usually diverged in 

 two opposite double rows 

 owing to the fact that 



Fig. 333. Median longisection of tip of primary ^^ originate at a slight 

 root showing histogens : w, cortex ; dgn-cal , derimt- i j U A 



ogen-calyptrogen ; en, endodermis ; ep, epidermis ; angle tO and On each Side 



/?^«?, periblem ; />f/, pericycle ; /»/, plerome ; /■ <r, root q£ ^^^ j-^q prOtOXylem 

 cap; stl, stele. 



points. 



Ontogeny of the Primary Root. — The development of the 

 primary root and the vascular anatomy of the seedling axis have 

 been investigated by Port (ix), who used the New York variety. 

 The ontogeny of the root is similar to Janczewski's (11) third 

 type, in which three histogens are differentiated that produce the 

 stele, cortex, epidermis, and root cap. (Fig. 333.) As noted 

 earlier, the histogens originate in the embryo and are well defined 

 four days after pollination of the flower. (Fig. 318, E, F, G.) 



The innermost histogen or plerome gives rise to a cylindrical 

 stele. The cells of this region divide in all three planes, but 



— r c 



