Primulas have a number of features which are useful in distinguishing between species. If you are trying to identify a primula that exhibits a rare feature, then the number of possible species is reduced. Key features include: style position, leaf vernation, flower annulus, and farina.
First let's review the parts of a flower.

The petals are the colored part of the flower which collectively make up the corolla. The anthers bear the pollen and are supported by the stalk-like filament and together make up the stamen. The pistil is collectively the stigma (pollen receptor), style and ovary (where seeds form). The sepals are modified leaves forming the calyx which protects the flower bud.  Primulas usually have 5 petals, 5 stamens and 5 sepals.

A feature of primulas is their heterostyly nature. Almost all Primulas have a pin form (where the level of the stigma is above the anthers) and a thrum form (where the level of the stigma is below the anthers). The best seed set comes from cross pollination of the two types. When planting out, group at least three or more seedlings to ensure you have both types growing together to encourage seed production. Hand pollination can also be used to increase your seed harvest.

This characteristic is useful in identifying a species when the suspected species is a homostyle type. This form has the anthers and stigma at the same level, and only occurs in a few species.

 

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Thrum flowered Primula polyneura
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Pin flowered Primula polyneura
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Homostylous Primula japonica
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Thrum flowered Primula auricula

Pin flowered Primula auricula

Newly emerging leaves show two distinct patterns of unfurling. The first have the leaf margins inrolled to the upper leaf surface and are termed involute. The second have the leaf margins rolled toward the leaf underside and are termed revolute. Recently, a third type of leaf vernation has been shown to occur in Primula. In this conduplicate form the leaf edges meet in the middle like a clam-shell. The characteristic is useful in identifying a species when the suspected species is either "involute" or "conduplicate" type as this only occurs in a few species.
 
Revolute leaf Revolute leaf in cross-section, P. involucrata

Conduplicate leaf, P. simensis

Copyright Pam Eveleigh 2007, All Rights Reserved