The sunflower seed is the fruit in the sunflower (Helianthus annuus). The phrase "sunflower seed" is really a misnomer when given to the seed in the pericarp (hull). Botanically speaking, it's much more properly termed as a achene. When Un-shelled, the edible remainder is termed the sunflower kernel.
There are actually a couple of sorts of popular seeds. Linoleic (most familiar), high oleic, and also Nusun. Any selection have their special levels of monounsaturated, saturated, and polyunsaturated fatty acids. The details in this post alludes primarily for the linoleic variety.
