Pachycaul

Pachycaul [ Botany ]	Dictionary of botanic terminology - index of names

A thick-stemmed tree, with particularly swollen, stout and ± succulent trunk and branches.

Pachycaul (From greek Pachy=Large and Latin Cauli = Trunk)

Pachycauls are trees, with particularly thick-stemmed trunks, often bottle-shaped, unbranched or sparingly branched and ± succulent, with a massive parenchymatous pith and cortex and relatively little secondary wood.
The pachycaul may be considered a not `twiggy' intermediate growing form between a standard tree and a true caudicifom.

In general succulents are plants that store water in their plant tissue, they may be loosely grouped as:

Stem succulents: (most of which are cacti) plants the have swollen, moisture-retaining stems.
(e.g. Pachypodium namaquanum)
Leaf succulents: Plants that have foliage but often lack a stem, whereas cacti and other stem succulents have a swollen stem but mostly lack leaves. (e.g. Echeveria laui or many vygies, i.e. members of the family Mesembryanthemaceae – commonly referred to as mesembs)
Root succulents: Plants the have swollen, moisture-retaining roots. (e.g. Pterocactus tuberosus)
Caudiciforms & pachicauls: Plants the have swollen, moisture-retaining trunks. . (e.g. Uncarina grandidieri, Adansonia digitata)
Bulbous: Plants that have swollen, moisture-retaining bulbs. (e.g. Ornitogalum caudatum)

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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