Ppz explai. With a diagram

Ppz explai. With a diagram Anatomically fairly old dicotyledonous root is distinguished from the dicotyledonous stem by [AIPMT (Prelims)-2009) (1) Absence of secondary phloem (2) Presence of cortex (3) Position of protoxylem (4) Absence of secondary xylem

Dear student, 
​Please find below the solution to the asked query        

Whenever we want to distinguish between stem and root, we should look for position of protoxylem. In roots, the position of protoxylem is exarch (protoxylem towards periphery, metaxylem towards center) while in stem it is endarch (protoxylem towards center and metaxylem towards periphery).  
Hence, option (3) is correct.

Diagram of Dicot root:

Diagram of dicot stem:


Hope this information will clear your doubts about the topic.    Regards