protcxj//e/n po/es 

 protop/?/oe/n po/es 



er/cj/c/e 

 mature xj//e/r? e/e/ner?£ 



cortex 

 ep/dier/n/s 



- e/?o J oa f er/7?/s 



/mmoti/re xg/e/77 e/ement 

 w/t/i secondary tva// 



SSCUl 



mature s/ere /i/6e 



/m/nature s/ere tude 



rootcap 



Figure 2. Diagram of a tobacco root tip, showing spatial 

 relations of different regions of the root and of the first vascular 

 elements. This illustrates the order of maturation of tissues in 

 the root tip. In this example the xylem matured at more than 

 twice the distance of the sieve tube from the initials. The en- 

 dodermis, as a cell layer, becomes defined before the sieve tubes 

 differentiate (see Figure 3A), but develops Casparian strips 

 slightly below the level where the first xylem elements mature. 

 In this figure the endodermis is indicated only at the levels 

 where Casparian strips are present. — Esau (23) 



