When
a leafy shoot is severed from a plant the flow of auxins (plant
hormones) from the tip to the base of the plant is interrupted,
and it accumulates at the base of the cutting promoting the
formation of roots.
Roots can form from ordinary cells, or can arise from existing root 'initials' commonly found around the nodes - where leaves join the stem. Cuttings are often severed just below a node to take advantage of this.
Although cuttings can form roots, they need to generate the material to form roots by photosynthesis. If cuttings wilt, stomata close and no photosynthesis occurs. Speedily transferring cuttings to a propagation area with a damp atmosphere, but good light levels, can prevent wilting. However, this can be at the cost of such high humidity that fungal rots damage cuttings and carbon dioxide becomes depleted. Without carbon dioxide photosynthesis does not happen. Judicious ventilation is therefore needed to shed surplus moisture and top up carbon dioxide supplies with fresh air.
Because leaves in sunshine are hot, they are surrounded by warmer air, which can hold more moisture than cool air. Hot leaves can lose moisture even in a humid atmosphere. Some shading to keep leaves cool is therefore needed, but not too much in case photosynthesis is reduced.
The rooting medium should be well drained and moist, but never wet. Root formation requires the cells to grow and divide. Air is necessary for this to happen, and soggy compost is poorly aerated.
Cell division and root formation is encouraged by warmth, 15-25oC (59-77oF), in the rooting zone. Root production can be suppressed above 25oC (77oF). However the atmosphere around the foliage should be kept cooler than the root zone. A warm atmosphere encourages shoots and leaves to form before sufficient root has formed to support the shoot.
In practical terms the environment required is provided by proprietary, or by custom-made propagators. These provide heat that is distributed to the pots and trays holding the rooting medium. They allow variable ventilation to prevent disease and drips and may use white plastic to avoid excessive heat build up.