Instant Notes: Plant Biology

epidermal cells but sometimes there is some in the mesophyll too.

There are three main types of pigment in petals. The most widespread are

flavonoids, which include at least three types of anthocyanins(Fig. 2) respon-

sible for the pink, purple and blue colors, and a number of similar anthoxan-

thinsresponsible for ivory-white and yellow colors which may be present in

addition to the anthocyanins. Betalains, similar in structure to alkaloids(Topic

J5), form intense purples and yellows in a few plant families, such as the cacti.

Carotenoidsoccur in some yellow flowers such as daffodils and marigolds.

Most flower-visiting insects have good color vision but are sensitive to a

different spectrumfrom us and other vertebrates, being insensitive to red but

sensitive to the near ultraviolet (UV). Floral pigments of insect-pollinated

flowers are more varied within this color spectrum than within that of verte-

brates, many flowers that are plain white to us absorbing in part of the UV spec-

trum, so an insect will see a range of shades. The lines towards the base of some

petals, known as nectar guidemarks, are exaggerated for insects since most

absorb in the UV range giving a darker center to the flower for an insect.

Stamens Most plants have four antherson each filament, some two (Fig. 3). The anthers
have an outer epidermis, and inside that a fibrous layerforming the bulk of the
wall, and an inner tapetumproviding the nutrients for the developing pollen
grains inside. When the anther is mature it dehisces, i.e. the wall splits to release
the pollen. In most plants it splits all the way down along a line of weakness, but
in some only the tip of the anther splits, and certain families have specialized
dehiscence. In some plants, such as nightshades, the pollen is only released when
a visiting bee vibrates its wings at a certain pitch (known as buzz-pollination). In

D1 – The flower 43

HO

HO

OH

OH

O+

Fig. 2. The molecular structure of an anthocyanin (cyanidin).

Line of

dehiscence

Flament

Line of dehiscence

Tapetum

Pollen grains

(a) (b)

Fig. 3. A typical stamen with four anthers: (a) surface view, (b) transverse section.