Midvein or primary vein or Midrib

Mid vein [ Botany ]
Synonym: Main vein, Midrib

Dictionary of botanic terminology - index of names

The midvein or primary vein is the main or central vein of a leaf from which arise the secondary or lateral veins.

More frequently called midrib or stem of the leaf, especially when it is prominently raised or depressed, the midvein is the main or central vein of a leaf from which arise the secondary or lateral veins. It is usually continuous with the petiole and is often raised above the lamina (the leaf blade). On a compound leaf, the midrib (rachis) extends from the first set of leaflets (where the petiole ends) to the end of the leaf.

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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