4XX THE STRUCTURE OF ECONOMIC PLANTS 



Casparian thickenings, and those abutting the phloem groups are 

 filled with a mucilaginous substance that is lacking in those 

 adjacent to the protoxylem. The cortical parenchyma is spongy, 



Fig. 115. A, transection of primary root; B and C, of lower hypocotyl ; D, of middle 

 hypocotyl : ca, cambium ; co, cortex ; en, endodermis ; ep, epidermis ; mx, metaxylem ; pel, 

 pericycle; ^^, phloem; pi, pith; />x, protoxylem. (After Spieth.) 



consisting of eight to twelve concentric layers of thin-walled, 

 isodiametric cells except for the two or three rows immediately 

 within the epidermis, which are axially elongated and more com- 

 pact. The epidermis consists of a single layer of small cells. 



