A plant hormone (otherwise known as a phytohormone or sometimes an Auxin or Cytokinin, although there are subtle differences between these) are hormones which help the plant grow. Some substances are able to trigger the germination of seeds. Others help plants grow in an ordered structured way, not too large, not too small, with the correct shape of all the elements such as leaf-shape and shape of leaf lobes, etc. This is governed by growth-inhibitors and growth promoters; together they create a plant growth regulatory system using negative feedback.


Some signalling molecules travel by air, others are transmitted underground (when the underground hyphae of fungi may aid transmission of the molecules) to signal to like plants nearby of a need to take action on account of either some on-going threat, or in response to other environmental conditions such as lack of water or the presence of un-desirable substances either in the air or in the ground.

Fungi are ever-present in soils, a bewildering number of them, many as yet un-identified. The hyphae of these fungi can stretch invisibly underground over large distances, connecting the roots of several species of plants, usually to the mutual benefit of both fungi and plant. The fungi can extract water and water-soluble nutrients from the plant this way, and the fungi can supply the plant with much-needed substance. Signalling molecules from plants can also travel via these hyphae to other like plants, giving them a warning of some external threat or environmental condition. These filaments are networked to a great many roots over what can be huge distances, but more often a few metres away.

Many Orchids are connected this way and cannot grow without their correct fungal partner, for some orchids can only obtain vital sustenance from the fungus. An example would be Ghost Orchids, which are white, totally lacking chlorophyll and the ability to photosynthesize their own food requirements. The seeds of some Orchids also connect to such hyphal filaments (the mycellium of the fungus) in order to grow. There is an un-seen information chemical super-highway just below ground that is not normally visible.


Phytoalexins are toxins produced on the spot by the plant in response to some on-going attack by a foreign organism, such as an insect, a virus, a bacterium or any other pathogen. The toxin is designed to either debilitate, disable, repel or kill the invading organism. In particular, the toxins are only produced when required and (to save the energy of producing the toxins) not produced when there is no on-going invasion.

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