Easton's Bible Dictionary: M. G. Easton, M.A., D.D.
The reed of Egypt and Palestine is the Arundo donax, which grows to the height of 12 feet, its stalk jointed like the bamboo, "with a magnificent panicle of blossom at the top, and so slender and yielding that it will lie perfectly flat under a gust of wind, and immediately resume its upright position." It is used to illustrate weakness (2 Ki. 18:21; Ezek. 29:6), also fickleness or instability (Mt. 11:7; comp. Eph. 4:14).
A "bruised reed" (Isa. 42:3; Mt. 12:20) is an emblem of a believer weak in grace. A reed was put into our Lord's hands in derision (Mt. 27:29); and "they took the reed and smote him on the head" (27:30). The "reed" on which they put the sponge filled with vinegar (Mt. 27:48) was, according to John (Jn. 19:29), a hyssop stalk, which must have been of some length, or perhaps a bunch of hyssop twigs fastened to a rod with the sponge.