There are about 25 species of Capsicum, most natives of South America, of which five have been domesticated. Most of our common chillis come from one species, Capsicum annuum, which was first cultivated in Mexico at least 5,000 years ago. Chillis are hollow fruits, with an outer wall rich in carotenoid pigments that encloses the seeds and the tissue that bears them, a pale, spongy mass called the placenta. (For chillis as vegetables). Their pungent chemicals, the capsaicins, are only synthesized by the surface cells of the placenta, and accumulate in droplets just under the cuticle of the placenta surface. That cuticle can split under the pressure and allow the capsaicin to escape and spread onto the seeds and the inner fruit wall. Some capsaicin also seems to enter the plant’s circulation, and can be found in small quantities within the fruit wall and in nearby stems and leaves.