Sunday, 12 March 2017

Crocus verna - an example of a species of family Iridaceae

The flowers of crocuses appears to come straight out of the ground and this is indeed the case. The ovary of the flower is actually below ground level at flowering time and unless you pull the flower up to include at least some of the below-ground structure you will probably miss the ovary altogether - ensure you only do this on crocuses from your own garden or if you have permission to do so.  Crocus verna is probably the commonest crocus grown in Britain, abundant in gardens.  It comes in a variety of colours, mostly purples and whites.

The genus name Crocus probably comes from ancient words for saffron.  One species is cultivated to produce saffron from the stigmas.  The species name verna is Latin for spring.

The leaves are long and narrow and the flower parts are in threes, indicating this is a monocotyledonous plant.

There are six perianth segments (not called petals because they are all similar and in two whorls) and these taper at their base and all combine to form a very long perianth tube which continues below the level of the soil. This situation where the perianth segments are united at the base is gamophyllous. The perianth tube is surrounded by a papery bract and there is a further bract below this which surrounds the perianth tube and the base of the leaves. Both of these bracts are attached to the stem below the level of the ovary.

The flowers open during the day and close at night. When open the division into three outer perianth segments and three inner ones is clear. Inside the bright orange stigma is divided into three main lobes which are themselves irregularly lobed.

Stripping back a couple of the perianth segments reveals some of the flower parts. There are three yellow anthers which are long and pointed, attached at the base to white filaments. The anthers are therefore basifixed. The anthers and the filaments together are called the stamens. Pulling back the perianth segments further reveals that the stamens are attached to the base of three of the perianth segments. In this case the stamens are said to be epiphyllous.



The anthers are divided into two sections, each one called a theca. To release the pollen they split from base to tip towards the centre of the flower. The splitting of the anthers is termed dehiscence and this type of splitting is called longitudinal dehiscence. Splitting on the side towards the centre of the flower is called extrorse dehiscence.



If the two bracts and several of the perianth segments are pulled back right down the perianth tube, the ovary is revealed. The stalk below the ovary is called the pedicel and this is the name given to the stalk of any individual flower.


If the ovary is cut using a blade it is found to be made of three hollow sections (locules) packed with ovules. If an ovary has three locules is it called trilocular. The ovules are attached to the central axis of the ovary by a stalk (the placenta). The point of attachment of the placentas is described as the placentation and in this case we have axile placentation.   There are tiny circles visible in the wall of the ovary and these are the vascular bundles.  There is one of these where the wall dividing the locules reaches the ovary wall and another around half way round the locule.

The following combination of characters will identify a plant to family Iridaceae:

  • long thin leaves
  • perianth segments all similar in two whorls of three
  • three stamens
  • style appearing single with three stigmas 

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