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Morphology of Cacti – Areoles

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The Cactaceae family is characterized by the presence of stems bearing specialized axillary or lateral bud or short-shoots or branch termed areoles, which usually develop the spines (modified leaves) that represent the familiar hallmark of this plant group (Taylor 1997, Mauseth 1983, Gibson and Nobel 1986). Areole is a vegetative growth which occurs on the raised portion of the stem is referred as tubercle or on a rib which is linear array of fused tubercles (Gibson and Nobel 1986).

The tubercles lack a furrow or groove, on the forward side, a character used to separate them from other tubercled genera such as Neobesseya and Coryphantha (Boke, 1952). In Coryphantha, Escobaria, and Neolloydia the distal floral portion is connected to the more basal spine-producing portion by a groove (Boke 1952, 1961). Mammillaria, some species of Ariocarpus, and Pelecyphora still have the separated basal and distal portions but lack the groove (Boke 1953).

Areole is a short shoot possesses nodes, the points of leaf attachment, and internodes, the portions of stem between adjacent leaves. The internodes are very short, however, creating a mass of compacted nodes, and the axillary bud associated with each leaf position gives rise to the spines and reproductive structures. When looking at both the areole and tubercle, one can often see that the areole is, in fact, borne on the enlarged leaf base or tubercle. Depending upon the number of meristems present, the areoles may be mono- or dimorphic which is an important taxonomic implicate.

Monomorphic: An areole with a single meristem, which may form a flower or a branch as well as spines, is said to be monomorphic e.g. Coryphantha sp. (Boke 1958).

Dimorphic: If there are two areole meristems, one of which produces only spines while the other forms a flower or a branch, the areole is said to be dimorphic.

Most areoles produce an indumentum or a covering of spines and multicellular hairs or trichomes, the latter often giving the areole a woolly appearance, as in Mammillaria hahniana and Rebutia marsoneri.

Meristematic activity: If an areole is examined microscopically, you will see that the areoles are bilateral in their symmetry, with the spines usually arising from the edges. 

The growing point of the shoot, the apical meristem, of most flowering plants produces growing points for leaves, the leaf primordia, all around the meristem area, spine primordia in the short shoot or young areole are aligned in rows on each side of the meristem (Mauseth 1983b, 273).

In some cacti such as Rebutia arenacea the areole is distinctly elongate but in other cacti it tends to be circular, for example, Gymnocalycium ragonesei.

The majority of cacti have areoles that produce spines or flowers for only a year or two, but a few genera such as Neoraimondia, Opuntia, and Pereskia may produce spines from individual areoles for many years (Mauseth and Kiesling 1997). Thus there is an increase in spine number at each areole over a period of years. This continued growth of an areole is particularly evident in the shoot like structures of cacti such as Neoraimondia arequipensis.

  

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