The Collaborative International Dictionary of English v.0.48:

Phyllotaxy \Phyl"lo*tax`y\ (f[i^]l"l[-o]*t[a^]ks`[y^]),
Phyllotaxis \Phyl"lo*tax`is\ (f[i^]l"l[-o]*t[a^]ks`[i^]s), n.
   [Phyllo- + Gr. ta`xis order.] (Bot.)
   The order or arrangement of leaves on the stem; the science
   of the relative position of leaves.
   [1913 Webster]