The Areole - Unique to Cacti ... 3/3

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Ribs and Tubercules

Cacti may present themselves as either ribbed where the aeroles are joined end to end in a continuous line, as in Echinocactus, Cereus etc, or tuberculate with distinct areoles like in Mammillaria. There is a number of intermediates in between where tubercules may line up and form rows, or conversely ribs may discontinue to form distinct tubercules at the extreme examples with many combinations between the extreme examples.
The young Echinocactus grusonii starts out tubercule like in it's juvenile form, before maturing into a ribbed specimen.

Tubercles range from long and leaflike - the extreme being Leuchtenbergia which looks more like an agave than a cactus (except for the long papery spines) and similarly some species of Ariocarpus hence the species agavoides - to broad and shallow like Turbinicarpus.