skin, and mesocarp, or flesh, fleshy in other members of Prunus such as the plum and cherry, is instead a thick, leathery, gray-green coat (with a downy exterior), called the hull. Inside the hull is a woody endocarp which forms a reticulated, hard shell (like the outside of a peach pit) called the pyrena. Inside the shell is the edible seed, commonly called a nut. Generally, one seed is present, but occasionally two occur. After the fruit matures, the hull splits and separates from the shell, and an abscission layer forms between the stem and the fruit so that the fruit can fall from the tree. Taxonomy Sweet and bitter almonds The seeds of Prunus dulcis var. dulcis are predominantly sweet but some individual trees produce seeds that are somewhat more bitter. The genetic basis for bitterness involves a single gene, the bitter flavor furthermore being recessive, both aspects making this trait easier to domesticate. The fruits from Prunus dulcis var. amara are always bitter, as are the kernels from other species of genus Prunus, such as apricot, peach and cherry (although to a lesser extent). The bitter almond is slightly broader and shorter than the sweet almond and contains about 50% of the fixed oil that occurs in sweet almonds. It also contains the enzyme emulsin which, in the presence of water, acts on the two soluble glucosides amygdalin and prunasin yielding glucose, cyanide and the essential oil of bitter almonds, which is nearly pure benzaldehyde, the chemical causing the bitter flavor. Bitter almonds may yield 4–9 milligrams of hydrogen cyanide per almond and contain 42 times higher amounts of cyanide than the trace levels found in sweet almonds. The origin of cyanide content in bitter almonds is via the enzymatic hydrolysis of amygdalin. P450 monooxygenases are involved in the amygdalin biosynthetic pathway. A point mutation in a bHLH transcription factor prevents transcription of the two cytochrome P450 genes, resulting in the sweet kernel trait. Etymology The word "almond" comes from Old French almande or alemande, Late Latin *amandula, *amindula, derived from amygdala from the Ancient Greek ἀμυγδάλη (amygdálē) (cf. amygdala, an almond-shaped portion of the brain). The al- in English, for the a- used in other languages may be due a confusion with the Arabic article al, the word having first dropped the a- as in the Italian form mandorla; the British pronunciation ah-mond and the modern Catalan