alligator pear avocado avocado pear black butter pear climacteric fruit cultivar egg-shaped fertility fruit firm food fruit green guacamole Hass variety healthy large berry large seed nutritious ovate pebbled skin perennials persea americana ripe rough soft tree wrinkled alligator-pear avocado-pear butter-pear climacteric-fruit large-berry large-seed pebbled-skin persea-americana kernel seed green-skinned hard unripened fertility-fruit flesh Fuerte-variety Hass-variety pear-shaped section smooth cut