The pleading of plaintiff shall consist of an original petition, and such supplemental petitions as may be necessary in the course of pleading by the parties to the suit. The original petition and the supplemental petitions shall be indorsed, so as to show their respective positions in the process of pleading, as "original petition," "plaintiff's first supplemental petition," "plaintiff's second supplemental petition," and so on, to be successively numbered, named, and indorsed.