Pumice

Pumice [ Geology - Horticulture ]

Dictionary of botanic terminology - index of names

A type of light-coloured porous volcanic rock, commonly used as an additive in potting mixes

Pumice is a frothy volcanic rock formed by the expansion of gas in erupting lava. Commonly perceived as lumps or fragments of pea size and larger but can also occur abundantly as ash-size particles. Because of its numerous gas bubbles, pumice commonly floats on water.

Pumice is an extremely porous and light-weight component of potting medias. Succulent growers frequently use pumice grit as an additive in potting mixes to create excellent drainage conditions for the plants. Although the pumice grit may be substituted by various mixtures and additives (e.g. perlite, vermiculite) the opinion of most cactus growers seems to be that pumice, where available, is the best aggregate to use as a basis for a good cactus and succulent mix, because it is loose and friable with lots of available space for air and roots. It drains very quickly, leaving air space while still retaining enough moisture to reduce frequency of watering without danger of root hair loss.. It is used in variable percentage and usually a potting soils for cactus contain at least 25-50% pumice, but plants demonstrated to grow also very well in a 100% pumice grit medium, with or without a small percentage (3-5%) of loamy soil or humus. This is particularly indicated for difficult species particularly sensitive to root rot.

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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