b o ta n y f o r G a r d e n e r s
Photomorphogenesis
low-light conditions.
Plant response to light that is neither directional nor
pollination and usually it is the intensity of their color
periodical is known as photomorphogenesis. It is how
that diminishes. This acts as an indicator to any passing
The color of most flowers fades after successful
light causes a plant to develop. An example is seen
pollinator that the flower has aged, that there is no
during germination, when the emerging shoot first
pollen or nectar reward, and that they should move on
encounters light. It will send a signal down to the root,
to another flower. In some plants the flower actually
causing the root to start branching. Plant hormones
changes color after pollination, as in certain members
are an important part of photomorphogenesis, as they
of the Boraginaceae family, such as forget-me-nots
are the signals that one part of a plant will send out to
(Myosotis) and lungworts (Pulmonaria), which change
initiate a response elsewhere. Examples might be tuber
from pink to blue. Color changes are also seen in
formation in potatoes, stem elongation in low light, or
fruit to indicate their ripeness.
leaf formation.
Color signals
“Touching” and “feeling” Plants are not only sensitive to touch, but they are also
Color is used by plants, often to trigger the senses
sensitive to other external forces such as gravity and air
of animals. No gardener can deny being attracted to
pressure. The directional response to touch is known
plants with plenty of large and colorful flowers.
as thigmotropism, and the response to gravity is
In the wild, colorful flowers are used to attract
known as geotropism.
pollinators, acting like shining beacons. Pollinators respond differently to the different
Thigmotropism
wavelengths of light, and flowers are colored
The tendrils of some climbing plants, such as species
specifically to attract their pollinators. Many insects,
of grape (Vitis), are strongly thigmotropic. Their
particularly bees, respond to long wavelengths of
tendrils feel the solid object on which they are
light in the blue, violet, and ultraviolet range,
growing, by detecting the contact via sensory
whereas plants predominantly pollinated by
epidermal cells called tactile blebs or papillae,
birds will have flowers colored red and orange. Butterflies prefer colors such as yellows, oranges, pinks, and reds. Many flowers are patterned with streaks or lines, called nectar guides. These serve as landing strips for insects,
which results in the coiling response. Any stems that twine around a support, or any clinging roots or twining petioles, are also doing so by thigmotropism. The plant hormone auxin once again plays an important role. The cells that
directing them toward their nectar or
have received the physical
pollen rewards. Some nectar guides are
stimulus produce auxin, which is
visible under normal light conditions, but many only show up under
transported to the growth tissue on the opposite
ultraviolet light. Fluorescence is also seen occasionally, perceptible in 180
Pulmonaria, lungwort