Some similarities to : Other
Violets especially Mountain Pansy (Viola lutea).
Hybridizes with : Wild Pansy (Viola tricolor) which itself comes in 2 sub-species: Wild Pansy(Viola tricolor ssp. tricolor) which has three colours (violet, white and yellow petals and Sand Pansy (Viola tricolor ssp. curtisii) which has bi-coloured petals, either white and yellow, or pale-blue and white. The hybrid between Wild Pansy and Field Pansy being called
Viola × contempta.
Your Authour thinks it very strange that books seem not to mention the square stems, which are square at least where the flower is.
Habitat : Farmers arable fields on light well-drained sandy soils and any place where the soils are disturbed regularly like gardens (except ours.
Wild Pansy is an archaeophyte and grows as a weed on arable land and on waste ground, to be found throughout most of the UK.
PROTECTION FROM EXCESS SUNLIGHT
A yellow to orange pigment,
Zeaxanthin diepoxide) [not to be confused with Vioxanthin, another coloured compound found in some Lichens) is present in Pansies (such as
Yellow Pansy). It is a di-epoxide xanthophyll, and it protects plants containing it from photo-oxidative damage by minimizing the formation of dangerous oxygen radicals. When the light absorbed by plants exceeds the capacity of the photosynthesis machinery to convert it to sugars etc, then the extra light goes towards de-epoxidizing violaxanthin to Zeathanthin in a 2-step process via an intermediate
antheraxanthin which has just a single epoxide group. This conversion is reversible - being called the violaxanthin cycle. This protective mechanism also operates in Thale Cress and other plants. The conversion of violaxanthin to zeathanthin is mediated by an enzyme called
violaxanthin de-epoxidase which is activated by the incidence of strong sunlight and the presence of
Ascorbate (Vitamin C) in a low pH environment. The effectiveness of this protective mechanism is thus limited by the availability of
ascorbate and if lacking for any reason then the plant may suffer stress under strong sunlight.
VioXanthin is yellow to orange (or yellow-green, depending upon whom you ask),
Antheraxanthin is bright yellow and Zeaxanthin is also yellow.
In low-light the reverse process happens, mediated by a different enzyme,
zeaxanthine epoxidase which adds the epoxide group to both Zeaxanthin and
antheraxanthin in readiness to protect the plant when the sun shines strongly again (up-arrows).
Antheraxanthin is an intermediate neutral-yellow carotenoid with just a single epoxide group (Violaxanthin possesses two; Zeaxanthin none).