cl_abap_structdescr finding data element descriptions of program defined structure

I have an program built structure (not ddic). with several fields, all defined with ddic data elements.

TYPES:
    BEGIN OF ty_selected,
      material       TYPE matnr,
      matl_type      TYPE mtart,
      lab_office     TYPE labor,
      plant          TYPE werks_d,
      status         TYPE mmsta,
    END   OF ty_selected.
DATA: gs_selected type ty_selected.

I want to retrieve the description text of the data elements that are in this structure. I just can't seen to get it.  I figured it out for ddic structures but not program defined ones.

For ddic structures I am using class cl_abap_structdescr like this:

DATA: ref_struct        TYPE REF TO cl_abap_structdescr,
      ls_struct         TYPE zmatl, " ddic type
      lt_ddic_info      TYPE ddfields,
      w_head            TYPE ty_return.
   FIELD-SYMBOLS:
     <ddic_info> TYPE LINE OF ddfields.

   ref_struct ?= cl_abap_typedescr=>describe_by_data( ls_struct ).
   lt_ddic_info = ref_struct->get_ddic_field_list( ).

   LOOP AT lt_ddic_info ASSIGNING <ddic_info>.
     MOVE <ddic_info>-fieldname TO w_head-fieldname.
     MOVE <ddic_info>-fieldtext TO w_head-fieldtext.
     APPEND w_head TO f_return.
   ENDLOOP.

I figure there should be something similar I can do for program defined structures, but no joy.

Can anyone help me please?

Thanks