Exploring Our Fluid Earth

Fig. 2.48. An indoor houseplant displaying positive phototropism toward a bright window light source

Image courtesy of VolodyA, Wikimedia Commons

Behavior can be defined as the responses by any whole living organism to stimuli. Stimuli (plural; singular: stimulus) are signals or inputs detectable by an organism. The term behavior is most commonly associated with active animals. For example, a shark is able to detect animal blood in water and might respond by swimming closer. However, aquatic plants and algae are also capable of exhibiting behavior in response to various stimuli.

Many photosynthetic organisms respond positively to light sources, for example, by growing towards the sun (Fig. 2.48). Phototropism is the growth response of an organism to light. The word phototropism comes from the Greek root words photo meaning light and tropos meaning turning.

Phytoplankton species capable of locomotion often exhibit positive or negative phototaxis. Phototaxis is the movement of an organism in response to light.  The word originates from the Greek words photo meaning light and taxis meaning arrangement.

Fig. 2.49. Pneumatophores produced by a grey mangrove tree (Avicennia marina) displaying negative gravitropism by growing upward, Bay of Islands, New Zealand

Image courtesy of Kahuroa, Wikimedia Commons

Some aquatic plant species also show gravitropic behavior. Gravitropism is growth behavior in response to gravity. Shoots growing from plant seeds typically display negative gravitropism by growing away from the force of gravity. This response often occurs in the absence of light as plant seeds germinate underground. Plant roots typically display positive gravitropism by maintaining growth in the direction of gravity.

The roots of some species of mangrove trees produce pneumatophores. Pneumatophores are specialized aerial roots that help the plant absorb oxygen in waterlogged sediments. They exhibit negative gravitropism by growing upward from deep lateral roots (Fig. 2.49).