Double flowering

Double flowering [ Horticulture ]	Dictionary of botanic terminology - index of names
Synonyms: Flore pleno, Plena, Pleniflora.

A plant species, cultivar, or variety having flowers with more than the normal number of petals, sepals or florets, such as roses. In some cases petals or staminodes may replace stamens.

The expression "double-flowering" and “semi-double flowering” does not refer to the number of flowers. Rather, the number of petals in each flower is at least double the number in the regular type.
Most native type of plant are usually "single-flowering" that means they have a defined and limited number of petals arranged in one row. As this native plant passed into cultivation several valuable double-flowering cultivar have been derived and selected by gardeners, which took advantage of a wild flower's natural tendency to develop petals from stamens and to become "double" (e.g. roses, carnation and chrysanthemums)
In fact some simple type of flowers changed their outer row of stamens into petaloids, which gradually acquired the colour and appearance of true petals developing a more or les tight rosette appearance.
Some double flowered form are charming, very different and highly appreciates as ornamental items and have become very popular. Newer double-flowering cultivars, will most likely cost more than traditional singles.
Some double-flowering types especially can be so top-heavy that they flop under their own weight unless supported

The Latin name for double flowering is "flore pleno" A double form may be also designated botanically “plena” or “pleniflora”

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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