It is impossible to explore the cactus world for long
without encountering the word areole. Beginners
are assured that all cacti have areoles, and the best way
to tell a cactus from any other sort of succulent is to
look for these localized areas from which arise spines
and leaves (if present), branches, flowers and fruits.
The uniqueness of cactus areoles is also stressed. The
cacti have some peculiar organs which will never be
found in other plants. wrote Karl Schumann concerning
areoles in his Keys to the Monograph of
Cactaceae: 2 in 1903. It is the purpose of this inquiry
to look at the evidence and see just how far these dogmatic
statements stand up to the test.
First we must consider the word itself and how it came
into botanical usage. Next, a look at a selection of areole
types in the Cactaceae and how they may have arisen
from other, less specialized ancestors. Some knowledge
of morphology and anatomy is called for, but not
too deep to affright timid readers. Finally we look
beyond cacti to representatives of at least 7 other families
that have independently evolved often surprisingly
similar organs. The exercise involved scrutiny and
close-up photography of living specimens matched by
A Hypothetical mesophytic
ancestor with buds in leaf axils
giving rise to shoots (or flowers)
B Spur shoot from telescoping of
axis; leaves diminishing; succulence
increasing
C Typical areole with hair tuft
partially sunken in the fat stem, and
subtended by a scale leaf, the leaves
of the spur shoot transformed to
spines
Figure 1
Presumed origin of a cactus areole
Lamina
(leaf blade) Petiole
(leaf stalk)
l Green dots indicate
potential meristems
Stipules
l
l
l
l
l
l l
l
l
l
l
l
l
l
l
l
l
l
ll
l
l
l
l
l
l
What is an areole?
Gordon Rowley puts a familiar term under the
microscope. Photography by the author.
Cactusville, 1 Ramsbury Drive, Earley, Reading, RG6 7RT, UK
Areoles are unique to cacti  Ted Anderson in The Cactus Family: 26, 2001.
All cacti have spine clusters, but no species of any other plant family does
 Mauseth, Kiesling & Ostolaza in A Cactus Odyssey: 81, 2002.
4 BCSJ Vol. 21 (1)
a search for previous literature, which revealed a vast
amount, mostly highly technical. Hence much has
been omitted and what is offered is but an appetiser
rather than the full meal.
Definitions
Areole is an anglicized version of the Latin Areola
meaning a little area or open space. General dictionaries
give a number of uses: a depressed spot, the space
between intersecting veins on a leaf, the polygonal surface
on a lichen, as well as zoological uses. Oddly
enough for a start, none of these botanical uses is as
popular today as a spine-bearing cushion of a cactus,
which is not even mentioned. Yet that has been in use
at least since the early nineteenth century. The earliest
reference I have traced so far is to De Candolle who, in
1828, described cacti in French and Latin. The French
text has faisceau (a tuft or bunch) and the corresponding
Latin areola. The term areola is freely used by
Frster in 1846 and 1884, and by Schumann in 1897,
the German equivalent being Areole, plural Areolen.
Haworth in 1830 (Phil Mag. N.S.7: 108) coined the
name spinarium for the peculiar fascicles of spines in
Cacteae, for which some writers use the term Area.
Unfortunately his rather apt word never came into general
use. Lemaire, who relished being different and
coining new words, introduced tyleole in 1868 (Les
Cactes: 15) meaning a small knob or lump, but again
nobody was listening.
Modern botanical dictionaries define areole as a space
marked out on a surface by cracks or ridges (Stearn,
Botanical Latin 1966) or more specifically for cacti as
a sunken cushion representing a condensed lateral
shoot from which spines, branches and flowers arise in
cacti ... (Penguin Dictionary of Plant Sciences 1999).
That can suffice as a starting point for the inquiry.
Interpretation
Take a deep breath and prepare for some basic botany.
All flowering plants share similar patterns of growth
and branching. Fig. 1A shows the typical features. We
say that each leaf subtends a bud in its axil; that is,
where there is a growth centre (meristem) it is protected
by the base of a leaf (and its stipules, if present).
From this plan we can trace many modifications.
Those of concern here relate to surface reduction and
succulence as a means of survival in increasing
drought.
Some familiar normal plants of temperate regions
such as pines and apple trees have two sorts of branches:
long shoots and short or spur shoots. The lat-
Figure 2 Stem apex of Matucana aureiflora, unusual among
globular cacti in having a visible scale leaf subtending each
areole when young
Figure 3 Ferocactus gracilis subsp. coloratus with typical cactus
areoles bearing hair, radial and central spines but no leaves
Figure 4 Neoraimondia arequipensis seedling. The areoles later
become fuzz-balls and, if destined to flower, spur shoots
BCSJ Vol. 21 (1) 5
6 BCSJ Vol. 21 (1)
ter have the axis shortened to bring all growth buds
close together, the cones or flowers emerging at the
top, and nowhere else. In the advance into drier and
drier terrain, leaves may be reduced in size and lost
altogether or transformed into spines, bristles or hairs.
This is possible only if the stems become green and
take over the function of the leaf blades. Fig.1B
shows a hypothetical intermediate stage with the spur
shoot and main axis becoming more succulent.
The cactus areole thus becomes the ultimate stage in
this telescoping (Fig.1C). The last leaf to go is that
subtending the original short shoot. A few members
of Cactoideae still retain this tiny green scale leaf
(Fig.2). All remaining leaves (bud scales) have become
spines and a tuft of hairs now protects the tender
growing points (two, basically) now set at stem level or
slightly below it (Figs.36).
In vertical section a typical cactus areole is shown in
Fig.7. Essential features are the two meristems: one
that early sprouts hair and spines, the other that bides
its time until awakened to grow a branch or flower.
There is a fascinating background story here: Boke
(1980) and Gibson & Nobel (1986) have the most
readable and accessible introduction to it. Spines may
develop quickly, or slowly, over a long period of time.
Tree-like species of Opuntia and Quiabentia go on producing
spines from their lower areoles even in old age
(Fig.8). Other cacti lose them. I once experimented
with a pan of seedlings of Cephalocereus senilis, shaving
half of them to see if they grew faster in our sunstarved
climate when the green tissues were exposed.
Results were inconclusive, but the shaved areas
remained bare, and in one plant even after 50 years the
area has only a few feeble, curly, white bristles.
Buxbaum (1950) distinguished two types of areole
according to their symmetry: radial, as in Selenicereus,
with spines spreading starlike around the growth cen-
Vertical section through an areole on the flower tube of
Selenicereus macdonaldiae (after Leinfellner 1937)
A = main axis
B = subtending leaf (scale)
G = vascular tissues
D = spines
H = hairs
S = growing point (meristem) Figure 7
Figure 5 In Echinocactus ingens the areoles are sunken beneath
a ridge of felt running down the ribs (Compare to Fig. 13)
Figure 6 Cactus areoles can be all felt and no spines
(Lophophora, left) or bear spines, glochids and felt (Opuntia
articulata, right)
BCSJ Vol. 21 (1) 7
tre, and unilateral where the growth bud is above the
spine cluster. This recalls the comparable situation in
flowers: actinomorphic (regular) or zygomorphic
(oblique-limbed). Where the areole sits on top of a
tubercle, further digressions can occur. In
Leuchtenbergia both spines and flowers arise from the tip
of a (young) tubercle. Split areoles separate the points
Figure 8 Quiabentia chacoensis showing the old areoles near the
base continuing to grow extra bristles
Figure 9 The unusual areoles of Mammillaria luethyi with many
microscopic spines on top of long cylindrical tubercles
Figure. 10 A halved mature Melocactus showing the terminal
cephalium of hair and bristles protecting the flower-producing
meristems (after Suringar)
Figure 11 Fuzzy spur shoots of Neoraimondia, that on the right
halved to show the origin of a flower (after Schumann)
8 BCSJ Vol. 21 (1)
Figure 12 Euphorbia didiereoides with 2 major (stipular) spines
and 2 or more additional spines surrounding each growth
centre. Note how the short lateral shoots arise
Figure 13 Euphorbia neohumbertii has conspicuous fur-lined
prickly ribs, but the growth centres are set in the valleys, just
visible as a tiny dot above the large leaf scar
Figure 14 Pachypodium lealii subsp. saundersii with a horny
shield bearing 2 stipular spines and the leaf scar between them,
with the latent growing point just above
Figure 15 Monadenium spectabile (left) and M. magnificum
(right) showing scattered stellate tubercles and large white leaf
scars, each subtending a growing point
Figure 16 Myrmecodia echinata seedling beginning to develop
plate-like spiny tubercles (clypeoli) supporting the leaf and
lateral meristem
BCSJ Vol. 21 (1) 9
of origin of spines and
flowers. The beginning of
this can be seen in
Thelocactus, and in
Coryphantha the flower is set
on the upper side of the
tubercle, sometimes on old
plants with a visible groove
running on to the spines.
Mammillaria completes the
separation by bearing flowers
in the usually woolly
bases of the spine-tipped
tubercles. Buxbaum postulates
an evolutionary series
from the radial areole of
Selenicereus to the unilateral
split areole of Mammillaria.
Epiphytic cacti usually lack spines, and have the small
areoles deeply sunken in the stem tissue with only a
fine brush of hair indicating their presence. In
Schlumbergera the active areoles are localized in a furrow
crossing the top of each leaf-like joint. Extraordinary
areoles are found in Mammillaria luethyi (Fig.9) with
around 80 minute stellate spines, and Blossfeldia where,
in keeping with the smallest cactus, they are only about
0.4 mm in diameter. At the opposite end of the scale,
areole proliferation leads to the lateral cephalia of several
genera of columnar cacti, and to the big terminal
cephalium of Melocactus (Fig.10). Only one of these
evolutionary excesses will
be mentioned further
here, because it is the only
cactus directly recalling the
spur shoots of its (presumed)
early ancestors.
Neoraimondia (Figs. 4,11)
has its very own fluff-balls
that, on flowering adult
branches go on growing
and blooming and can
reach 9cm long, sometimes
with two heads.
Buxbaum defined the areole
as having typically two
meristems: one generating
spines, the other branches
or flowers. However, it is
clear that more growth
centres may be present, as shown by some cacti developing
two or more small flowers from a single areole
(Rhipsalis, Eriosyce).
Other families of succulents
Many plant families have responded to problems of
water economy in similar ways to the Cactaceae, evolving
water-storing tissue (succulence) and reduction of
exposed surfaces to such an extent that they may end
up looking and functioning surprisingly like cacti. It
needs the flowers and fruits to confirm the difference.
It is not surprising, therefore, to find that many have
spur shoots of a sort, or even greater telescoping to
Figure 17 The tessellated tubercles of Euphorbia
gymnocalycioides. The dark spot is the scar from a subtending
scale leaf. Above it are scattered inflorescence scars and
white scales
Figure 19 Fouquieria fasciculata with spur shoots arising in
the axil of a leaf, of which only the hardened dry spiny
petiole persists
Figure 18 Sarcocaulon vanderietiae with spur shoots that could
be compared to the areoles of Pereskia
10 BCSJ Vol. 21 (1)
lead to structures resembling cactus areoles. It is fascinating
to pass some of these in review and try to work
out how they came about: different solutions to the
same problems of protecting tender growing apices
and conserving water. At least 7 families are involved.
Armature may arise from hardened remains of lost
leaves, as in cacti, from leaf stalks, from stipules or
from woody remains of shoots or inflorescences.
Leaves may be wholly lost, or solitary (subtending the
spur) or of two types according to siting on the main
axis or within the spur.
Euphorbiaceae
Euphorbia is the genus that shows the greatest range of
cactus-like modifications, both in habit and concentrated
growth centres. All the above-mentioned types of
armature occur. The leaf axil may bear two lateral
spines (derived from stipules) or a central thorn
(derived from a stem) representing a skeletal branch
system (Mellichamp 1994). But they arise from a
horny shield, not a felt cushion as in cacti. Scattered
surface prickles are a feature of many Madagascan
euphorbias: in E. didiereoides (Fig.12) the two lower
small spines are stipular, the assorted upper outgrowths
are prickles. Other contenders for pseudo-areoles
with more than two spines are E. dasyacantha, E. horrida
and E. whellanii. E. neohumbertii (Fig.13) has straight
ribs bearing fur and spines with the leaf bases and
growth centres quite separate between them.
Numerous other designs could be mentioned, but one
must suffice: one of the most extraordinary of all, the
aptly named Euphorbia gymnocalycioides (Fig.17). This
has humped tubercles like a Gymnocalycium, each divided
into a lower half with a conical tip (the leaf scale
plus two minute stipular prickles when young) and an
upper half with a loose cluster of 15 meristems that
later grow out as inflorescences plus 28 white dots of
uncertain origin. Altogether it looks like no other
Euphorbia, or indeed any other plant, and features of its
anatomy, including the white dots, are still a mystery.
The related genus Monadenium has in some species
(M. spectabile and M. magnificum, Fig.15) cactus-like
features: tubercles topped with a crown of short prickles.
These are more or less randomly scattered over the
surface of the stem and are not directly associated with
the growth centres.
Apocynaceae and Rubiaceae
Another approach to the cactus areole is found in
Pachypodium (Fig.14) where two strong stipular spines
enclose the growing point, with the leaf scar below
and sometimes accessory spines of superficial origin.
A variation on this theme may be seen in the low,
plateau-like tubercle of some species of Myrmecodia
(Fig.16), caudiciform epiphytes from Malaysia.
Geraniaceae and Fouquieriaceae
Clear evidence of spur shoots in reduced form can be
seen in species of Sarcocaulon (Fig.18) and Fouquieria
Figure 20 Othonna euphorbioides challenging the cacti in areole
production. Note the thorns that may fork or retain shrivelled
floral remains
Fiure 21 Didierea trollii (left) and D. madagascariensis (right)
areoles flush with the stem or elevated on tubercles
BCSJ Vol. 21 (1) 11
(Fig.19, Killingbeck 1996). Here there is a single
spine developed from the hardened petiole of a leaf,
the green blade having served its purpose and been discarded
earlier. This subtends not one bud but a cluster,
suggesting a telescoped shoot. Approximation to a
cactus areole comes closer with the presence of bits of
persistent chaffy scales, but no actual wool.
Asteraceae (Compositae)
A single member of the huge daisy Family, Othonna
euphorbioides, goes a step further with a starlike cluster
of thorns (Fig.20) that persist around the growth centre
after the leaves have fallen. These thorns vary in
number and are occasionally forked. They represent
old or abortive inflorescences, and one can sometimes
see remnants of flower heads at their tips.
Didiereaceae
Finally we come to Didiereaceae, an ally of Cactaceae
and nicknamed the cacti of the Old World. In the two
species of Didierea (Fig.21) we find whorls of radial
spines (but no central spine) surrounding the meristem
with a fuzz of withered remains of the long, narrow,
deciduous leaves. In D. trolli the spur shoot forms a low
or hemispherical mound, but in D. madagascariensis each
is elevated on a slender tubercle recalling many cacti.
Summing up
We have briefly surveyed some of the strategies whereby
succulents protect their tender growing points
against desiccation and predators and cut water losses
by condensing and modifying spur shoots. Plants of
different Families have solved the problem in different
ways, but the end products often look surprisingly similar.
They are morphologically different but functionally
similar.
Purists will continue to use the term areole for
Cactaceae alone, although the only unique feature
about the cactus areole seems to be the universal presence
of hairs. But does this mean that we need further
terms for other sorts of growth centre? (Podarium is
already in use for the swollen leaf base of euphorbias).
Alternatively, should we extend the usage of areole to
cover Didierea, Othonna euphorbioides and perhaps other
kindred sorts? I confess to having already written of
areoles in Didierea, and can see no objection to wider
use of the existing term rather than splitting and
inventing new terms for similar organs in Families
other than Cactaceae. On this basis a refined definition
might be:
AREOLE: In xerophytic succulents a localized area of
stem with one or more growing points and often
spines, bristles, scales or hairs; a reduced spur shoot
initially subtended by a foliage leaf or scale leaf.
Excluded from this definition would be outgrowths
such as the star clusters of Monadenium or the fur-lined
ribs of some euphorbias, and the even more cactus-like
leaf diadems of Trichodiadema (Fig.22).
FURTHER READING FOR ZEALOTS AND POLYGLOTS:
BOKE, N.H. (1980) Developmental morphology and anatomy in
Cactaceae. BioScience 30: 605-610.
BUXBAUM, F. (1938). Allgemeine Morphologie der Kakteen. Cactaceae
Jahrbcher der Deutschen Kakteen-Gesellschaft Okt.1938 (unpaginated).
[Of historical importance.]
BUXBAUM, F. (1950) Morphology of Cacti, Ed. E.B.KURTZ, Pasadena.
Section I  Roots and stems.
BUXBAUM, F. (1964) Was ist ein Cephalium? Kakt. u.a. Sukk.15: 28-31;
43-48.
GIBSON, A.C. & NOBEL, P.S. (1986) The Cactus Primer. Harvard Univ.
Press.
KILLINGBECK, K.T. (1996) Unique formation of spur branches in
Fouquieria. Cact. Succ. J. (U.S.) 68: 267-268.
KRAINZ, H. (1957) Die Kakteen. II Die Areolen. Morphologie (5).
LEINFELLNER, W. (1937) Beitrge zur Kenntnis der Cactaceen-
Areolen. Osterr.Bot.Zeitschr. 86: 1-60.
MAUSETH, J.D., KIESLING, R. & OSTOLAZA, C. (2002) A Cactus
Odyssey. Timber Press, Oregon. [Incidental anatomical notes.]
MELLICHAMP, L. (1994) Are you stuck on the fine points of sharpobject
nomenclature? Cact. Succ. J. (U.S.) 66: 208-213.
RAUH, W. (1956) [Didiereaceae tubercles and armature] Abh. Math. Nat.
Klasse (6): 345-444.
RAUH, W. (1979) Kakteen an ihren Standorten. Berlin & Hamburg. [Die
Areolen pp.17-20; Die Mamillen pp.20-22.]
RAUH, W. (1985, 1987) Madagascan euphorbias: life and growth
forms. Euphorbia Journal 3: 18-37; 4: 11-26.
Figure 22 Cactus mimic Trichodiadema densum.
Each spine-tipped tubercle is a leaf; the stem is not succulent