question - plpgsql and query on table given by variable

Please, help me.
I am new with PostgreSQL, I need create function in plpgsql where I use SQL question on the name of table whitch is given by variable in called function. Here is absolutly simple example:

CREATE OR REPLACE FUNCTION pokus(varchar) RETURNS int4 AS '
DECLARE
v ALIAS FOR $1;
BEGIN
SELECT AsText(the_geom) FROM v;
RETURN 0;
END;
' LANGUAGE plpgsql;

And this is error:

SELECT pokus('sometable');
ERROR: syntax error at or near "$1" at character 32
QUERY: SELECT Astext(the_geom) FROM $1
CONTEXT: PL/pgSQL function "pokus" line 4 at SQL statement
LINE 1: SELECT Astext(the_geom) FROM $1
^

Also I tryed create function foo() whitch returns some varchar (in my case name of table) and then i tryed call it in other function like name of table. Example:

CREATE OR REPLACE FUNCTION pokus1() RETURNS int4 AS $$
BEGIN
PERFORM AsText(the_geom) FROM foo();
RETURN 0;
END;
$$ LANGUAGE PLPGSQL;

And I received this errors:
SELECT pokus1();
ERROR: column "the_geom" does not exist
CONTEXT: SQL statement "PERFORM AsText(the_geom) FROM foo()"
PL/pgSQL function "pokus1" line 2 at SQL statement

The column the_geom EXIST inside the table which is represented by my function foo(). When I displace foo() by same name of the table it works.

Thank you for your help.