set value form element with dynamic name

Hello, I have a dymanic generated form with elements like this:

<SELECT NAME="SECCION" onChange="CalcPlanta(<%=RSLISTA("ORDEN")%>)">
<OPTION VALUE="1">1.- Autores Españoles
<OPTION VALUE="2">2.- Clasicos Grecolatinos
<OPTION VALUE="3">3.- Poesia
<OPTION VALUE="4">4.- Literatura de 5 a 8 años
</SELECT></TD>
<INPUT NAME="PLANTA_<%=RSLISTA("ORDEN")%>_A" TYPE=TEXTBOX MAXLENGTH=2 SIZE=2>

But the function CallPlanta doesn´t work, because it can´t traslate de dynamic name of the element

<SCRIPT LANGUAGE="VBScript">
<!--Sub CalcPlanta(linea)
Dim TheForm
document.PEDIDO['PLANTA_' & linea & '_A'].value=1
End Sub-->
</SCRIPT>

How can i set the value of a element if I only know how "built" its name?
THANKS and sorry for my English