Definify.com

Webster 1913 Edition

Adj.

[

Penta-

+ Gr.

στίχοσ

a row.]

(Bot.)

Having, or arranged in, five vertical ranks, as the leaves of an apple tree or a cherry tree.

pentastichous ‎(not comparable)

(botany) Having, or arranged in, five vertical ranks.
The leaves of apple and cherry trees are pentastichous.