Groove

Groove [ Botany - Anatomy ]	Dictionary of botanic terminology - index of names
Adjective: Grooved (Fissured)

An elongate and fairly uniform narrow depression on the surface of an organism or on a bodily structure or part

Areolar groove [ Botany ]
Synonyms: Tubercle groove (Cactus anatomy)

An areolar groove is an adaxial extension of the areole on the upper side of the tubercle (from its base to its apex) found in several genera of cacti.

The areolar groove on the tubercles of Ariocarpus kotschoubeanus

Areolar grooves are found on tubercles of sexually mature cacti belonging to many genera (e.g Coryphantha, Ariocarpus, Thelocactus, Escobaria, Glandulicactus, Ancistrocactus etc) This plants have grooved and groove-less tubercles, the latter occur on juvenile plants and emergent branches and may occur mixed with grooved tubercles on adult shoots. The areolar meristem in the groove has a vegetative portions, usually spiny at tubercle apices and a reproductive portion (flowering or fruiting) portions in the axils of the tubercles but the 2 parts remain connected by a band of trichomes on the adaxial side of the tubercle (e.g., Coryphantha). Grooved tubercles have monomorphic areoles and are the only ones which bear flower. Several genera have a short groove reaching only part way to the axil of the tubercle, with the flower produced at the adaxial end of the extension. On the contrary in some genera (e.g., Mammillaria) the connecting groove is absent; the axillary reproductive regions are separate areoles distinct from the spiniferous vegetative areoles. (dimorphic areoles on grooveless tubercles)

In some species nectary glands occur in the groove of most tubercles and may occur at any position along an areole groove. The glands initiate by the areole meristem in the same manner as spine, with which they are homologous.
Areolar groove are also present in Ariocarpus:
A. fissuratus and kotschoubeyanus has monomorphic areoles with both floriferous and spiniferous areolar areas developing within the same meristem and never splitting off. A. retusus and A. agavoides, on the other hand, has areole dimorphism with the original single growing point becoming elongated and then dividing into two portions. However, A. trigonus, differs from both the above groups as the spiniferous growing point never develops from the original meristem. All species begin with an original meristematic area, with differentiation occurring during elongation and maturation of the tubercle. It is believed that this species have areolar meristematic areas with potential floral and spiniferous areas, but that growth and maturation can occur in one of three places in relation to them.

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Holdfast roots [ Botany ]
Dictionary of botanic terminology - index of names

Some species of climbing plants develop holdfast roots which help to support the vines on trees, walls, and rocks. By forcing their way into minute pores and crevices, they hold the plant firmly in place.

Climbing plants, like the poison ivy (Toxicodendron radicans), Boston ivy (Parthenocissus tricuspidata), and trumpet creeper (Campsis radicans), develop holdfast roots which help to support the vines on trees, walls, and rocks. By forcing their way into minute pores and crevices, they hold the plant firmly in place. Usually the Holdfast roots die at the end of the first season, but in some species they are perennial. In the tropics some of the large climbing plants have hold-fast roots by which they attach themselves, and long, cord-like roots that extend downward through the air and may lengthen and branch for several years until they strike the soil and become absorbent roots.

Major references and further lectures:
1) E. N. Transeau “General Botany” Discovery Publishing House, 1994