Here is a word that ought to be more useful than just
describing over-lapping leaves. I thought it might be an
astronomical term, meaning the line produced by the middle of something.(equi- = middle)
Date: Tue Jul 29 00:01:11 EDT 2003
Subject: A.Word.A.Day--equitant
equitant (EK-wi-tuhnt) adjective

Straddling; overlapping, as the leaves of some plants, such as irises.

[From Latin equitant-, stem of equitans, present participle of equitare
(to ride), from equit-, stem of eques (horseman), from equus (horse).]

A picture of equitant leaves:
http://www.life.uiuc.edu/plantbio/digitalflowers/Iridaceae/5.htm