Oktzim, Chapter Three, Mishnah Seven
1) The heart of a palm is like wood in every respect, except that it may be bought for [second] tithe money.
2) Unripened dates are considered food, but are exempt from tithes.
Section one: Heart of palm is not generally considered food and therefore it is not susceptible to food impurity. This is probably because often it wouldn’t be eaten, since the palm grew before the heart could be harvested. However, since it is edible when the palm is still young, one can use second tithe money to buy it.
Section two: Unripened dates are almost an opposite case. Whereas the heart of palm begins as food but then becomes inedible, unripened dates are currently inedible but will become edible later. They are considered food as far as impurity goes, but if one eats them before they are ripe, he need not tithe them.