vii: The Chemistry Column
Sensing sunlight Claire Gormley I am dreaming of warmer, sunnier days; reading in the park, the sun and a soft breeze on my shoulders, the sounds of children playing, the texture of grass on my toes, the smell of barbeques. Our incredible nervous system allows us to experience the world in so many different ways, and to recall these detailed sensations years or even decades later. Our nervous system is made up of two parts: the central nervous system and the peripheral nervous system. The central nervous system consists of our brain and spinal cord— the two most important structures for controlling our bodily functions, and retaining our memories and consciousness. The peripheral nervous system consists of receptors— sensory neurons, intermediate neurons and motor neurons —which together enable us to detect and respond to stimuli; a hot plate, a bright light, or a bird’s song. This is the part of our nervous system that actually allows us to experience and engage with our surroundings. While many other organisms have a nervous system, none are believed to be quite like ours (although it is currently being debated if some molluscs have consciousness as we know it). But what is clear from studying the nervous systems of other organisms is that
sensing the world around us is critical for survival. So how do organisms that don’t have a nervous system— like plants —experience and engage with their surroundings? One sure answer is heliotropism. Heliotropism is the movement of a plant with the sun during the day (Vanderbrink et al, 2014). It is also referred to as ‘solar tracking’ and is performed by numerous plants— most notably Sunflowers (Helianthus annuus) —in order to maximise photosynthesis, and thereby plant growth (Sherry and Galen, 2002; Vanderbrink et al., 2014). In contrast, phototropism— the better-understood phenomenon —is when a plant grows towards a fixed light, resulting in the sustained curvature of a plant. Think of the oddly shaped branches of young trees searching for light under a crowded canopy of mature trees. Technically, heliotropism and phototropism are the processes of plants responding to their surroundings, but I think the dynamic and continual movement of heliotropic plants draws a stronger parallel to how our peripheral nervous system engages with its surroundings. Feel free to make up your own mind… The mechanism that causes a plant to track the sun can be either turgor-mediated or growth-mediated (Vanderbrink et al, 2014). 39
