Tuesday, 14 March 2017

Prunus spinosa - a member of family Rosaceae

When the blackthorn is in bloom, spring is definitely heralded, preceding by a couple of weeks the more showy blossoms of the cultivated cherries (also genus Prunus) in towns.  My previous two studies have been of families of monocotyledons and so a change to a dicotyledonous family is due as I explain more botanical terms in a straightforward way.

Prunus is the Latin word for plum tree, while spinosa refers to the spines that are present.  These spines are often quite difficult to find, but you can find shoots that have long spines extending from the dark brown to black woody shoots. 

The flowers open before the leaves in the spring, but the flowering period is quite long so in a few weeks there will still be flowers along with leaves on the shoots. 

The flowers of the blackthorn are produced on the previous years’ shoots and arise very close to the main stems on short side shoots.  These shoots have a bud at the tip which opens to reveal leaves (vegetative bud) and the flowers arise from the base of the bud.  There may be one, two or three arise from the same level.

There are five white sepals at the end of the pedicel (flower stalk) and five white petals. These are called sepals and petals because they differ greatly in structure and colour.  The five sepals together are collectively called the calyx and the five petals together are the corolla.  The sepals are not attached to one another and are described as being free. The same is true for the petals.  There are 20-25 stamens and these consist of long white filaments and small yellow anthers, turning orange and then brown.  In the centre of the flower is a single green style with a single yellow stigma.  The stigma is rounded (termed capitate). In between the base of the filaments and the style is a glistening area which is an indication of nectar.

If the flower is cut in half with a blade it becomes clear that the sepals, petals and stamens are all attached to the top of a cup-shaped structure which has the ovary attached at its base.  This structure is called the receptacle. In other flowers this may be flat or of other forms but this cup-shape is quite common in the rose family.  The colour of this cup shape is golden and glistening and has glands on the surface producing nectar; it is described as being a nectariferous disc.  The stamens, petals and sepals aren’t attached to the top of the ovary or below the ovary, but rather on the rim around it.  This arrangement is called perigynous (peri = around; gynous = female parts).  The ovary however is described as being superior as it is itself attached to the base of the receptacle.  If it is cut across, it can be seen there are two ovules in the ovary.  They are attached to the top of the ovary and are said to be pendulous.

After fertilisation one of the ovules aborts and the ovary swells into a spherical fleshy fruit containing a single stony seed.  A fleshy fruit of this type with a hard stone in the middle is called a drupe. The fruit matures from green to dark blue with a whitish “bloom” that comes off when rubbed with a finger. These are the familiar sloes, collected for jam and gin.

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