There are quite a number of tuberous cacti, so we can indeed consider them to be in a family of bulbs. Even large cactus seeds need light to germinate properly. Almost all cactus seed floats, which perhaps aids in dispersal, and not much floating seed is buried for sprouting in habitat. Uebelmannia gummifera sprouts and grows under white quartzite sand for several years before emerging. Its tiny seeds easily fall through cracks in adjacent sand particles, and its habitat on a slope of sand ensures there will be no flooding to carry seed to a less favored substrate. The sand transmits a lot of light, which one may see by putting a few millimeters of such sand in a transparent dish and looking up through it at a light. Along with light, another requirement for cactus seed sprouting is constant wetness, not dampness. In this cacti are also like dry-area bulbs. Leo Martin