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The presence or absence of areoles determines whether or not a species is a cactus, i.e. belongs to the family Cactaceae.  Fig. a shows an example of the fairly large (ca. ΒΌ"/6 mm diam.) areoles of Trichocereus candicans: a dense mat of fibres, often referred to as 'hair' or 'wool', from which nine, aggressively sharp spines project.  Areoles are not always so large, intimidating and obvious as those of Trichocereus.  Zygocactus truncatus for instance, has much smaller, elongated areoles (Fig. b) with very fine hairs; the spines are represented by a few innocuous bristles (arrow).  The important thing is not the size or shape of the areoles, but the fact that they are present.

While the spines of Trichocereus are intimidating, they are also very obvious, giving out a clear 'keep away !' message that you couldn't fail to notice.  Compare this with Opuntia, where the areoles are equally aggressive, but in a much less obvious, and more insidious way.

Contrast all this with the spines of Agave americana (Fig. c).  They show no trace of anything remotely resembling an areole, which is why the Agave is not a cactus.



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