In some species (e.g. in Stellaria and Cerastium ) the petals are deeply cleft into 2 lobes: with a supeficial observation one could think that the petals are 10 and not 5, as it actually is, but when we exctract a single petal from the corolla we can realize their exact number:
In the species whose petals are fused together it usually easy to appreciate their number by counting the number of teeth (or lobes) of the corolla. When, however, the petals are completely fused, with no obvious teeth, it may be difficult to estimate their number:
In several species which have bilaterally symmetrical flowers with petals fused together, the corolla is profoundly modified, and the estimation of the number of petals may be difficult. In such cases we suggest to skip the character 'number of petals' if you are in doubt:
In the family Asteraceae the flowers are crowded in inflorescences called 'heads': the single flowers are of 2 types: tubular (regular) flowers where the corolla has 5 obvious teeth (and hence 5 petals) and strap-shaped, irregular flowers where the number of teeth is variable. We suggest to skip the character 'number of petals' for the species of this family.