The commonest that gardeners will meet is thigmotropism, also known as haptotropism, which involves sensitivity to touch. You have all seen beans and peas attaching themselves to a pole. This is an example of thigmotropism that gardeners use to support their plants. What happens is that tendrils respond to contact by curling round the supporting plant. It is also the case that Convolvulus [bindweed, which is a common weed on allotments] attaches itself to plants in the same way. Roots display negative thigmotropism when they move away on contact with a stone.
Chemical tropism is common and has several different types. The basic process involved in all is response of the plant to ions moving in the soil.
One form of tropism commonly known to gardeners is hydrotropism, which is a movement in roots towards a water source. Movement towards dampness is known as hygrotropism and is just a form of hydrotropism. The classic example of hydrotropism is the carrot. Carrot seedlings throw down a long taproot towards a deep water source, which is a legacy of carrots' origin as a desert plant. Growers often use this tendency to enable them to grow long carrots by growing in tubes which are watered from the bottom.The long roots that search for water thicken out to produce long carrots. This form of tropism is also found in tomato roots, parsnips and radishes. . The mechanism of hydrotropism is thought to be connected with the presence of abscisic acid in plant roots.
In arid conditions hydrotropism counteracts geotropism because arid conditions diminish the supply of amyloplasts and statocytes, thus weakening geotropic forces without lesening hydrotropic ones. It is also known that plants
Chemotropism is a response to chemicals in the soil. Many plant roots grow towards useful minerals and away from harmful acids.Roots in some plants cam display positive chemotropism in one temperature range and ngeativ chemotropism in another. A minor form of chemical tropism is aerotropism, in which plant roots grow towards an oxygen source,
A vital instance of chemotropism occurs in pollination, as the pollen's landing on the stigma, the plant's female organ, elicits a response from the ovule, which causes a pollen tube to develop down which the male pollen travels to fertilise the ovule
A final kind of tropism is thermotropism, which is movement of a plant towards a heat source. This can be found in house plants which are drawn towards radiators, bananas being an example. But outdoors this tropism also operates. It is well-known that maize likes a warm stem, but cool roots, and it has been noted that if there is any temperature incline in the soil in which maize is grown its roots will grow towards the cooler side,though this does not operate in cold conditions unsuitable for maize.
Knowledge of common tropisms is useful to gardeners. We do not need to be thinking about it all the time, but it is a weapon in the gardener's armoury of knowledge.