There are many types of cacti, and collectively they have many types of seeds. There are large seeds, oval seeds, kidney-shaped seeds, shiny and dull seeds, etc. They represent the diversity of cacti and their features help identify the various species just as spines do.
(Photo: Brightly colored saguaro [Carnegiea gigantiea] fruit with seeds, Ken Bosma)
M Rojas-Aréchiga and C Vázquez-Yana published a review of cactus seed germination a few years ago. Their introduction to the paper says it covers many topics.
Seed distinctive features are mentioned, such as colour, form, and size. Aspects of seed physiology, such as germination and dormancy, as well as seed dynamics including dispersal, predation, and soil seed bank formation, are included in the discussion. Techniques of propagation and some aspects of longevity and conservation are mentioned.
There are several important parts of seed biology that affect germination including 1. seed predation, 2. seed dissemination, 3. seed dormancy, and 4. microclimate.
Seeds are often eaten along with fruit by lizards or small mammals. Ants also feast on seeds, often ants carry seeds back to their nests. Seed predation can affect seed germination; seeds may actually germinate more easily after passing through an animal. Alternately, seeds may be stored cool and dark in an ant nest until a rainstorm releases the seeds.
Seed dissemination is helped by animals, and the new plantlets are distributed away from the mother plant–a good thing. But, some seeds are actually thought to be carried by water as a typical part of their dispersal. Some of these species grow in South American river valleys, such as Matucana.
It is known that some seeds seem to have an internal timer–they will not germinate for a year or more after development even if conditions are just right. Other seeds need heat and light to germinate. Still other seeds germinate in darkness. Thus, it seems some seeds must be buried in the soil and some not.
Seeds only germinate (and survive) when they have the good fortune to be in the right microclimate. Perhaps they are in the shade or a rock or under a larger plant. Most cactus seeds will not survive after germination if they are exposed to hot burning sun.