59^ 



THE STRUCTURE OF ECONOMIC PLANTS 



The Primary Root. — The primary root has a tetrarch, exarch 

 protostele which is surrounded by a pericycle that is uniseriate 

 outside the four primary phloem groups, but may be four or more 

 cells in width at the protoxylem points. (Fig. 3 10.) The cortical 

 region is limited centripetally by endodermal cells that are verti- 

 cally elongated, being about equal in length to those of the peri- 

 cycle. The remainder of the cortex consists of seven to ten layers 



-hyp cot- CZZ^j> ^hp s c . /T\ — hyp 



■ rt peg — A 



^1 ^ C 



A 



Fig. 309. Stages in development of seedling: A, two-day seedling showing axis prior to 

 formation of peg (seed coat has been removed) ; B, at three days, showing development of 

 peg; C, same, at five days, illustrating function of peg in splitting seed coat and aiding in 

 withdrawal of cotyledons; D, at eight days, primary growth of seedling is complete; E, 

 older plant showing position of first foliage leaves and relative lengths of first and second 

 internodes: cot, cotyledon; eel, epicotyl ; hyp, hypocotyl ; ino i, first internode; ino i, 

 second internode; / i, first foliage leaf ; / z, second foliage leaf ; rr, primary root ; j c, seed 

 coat. (After Whiting.) 



of large parenchymatous cells which are several times as long as 

 wide, and there are conspicuous intercellular spaces. The elon- 

 gated, tabular epidermal cells are small in radial dimension, and 

 each may form a root hair. 



The ontogeny of the root follows Janczewski's (xo) fourth type, 

 which is characteristic for the Cucurbitaceae and Leguminosae. 

 In this type, the growing point is not differentiated into distinct 

 histogens; but, instead, the primary tissues of the root axis arise 



