Outside my window the trees are beginning to take on their autumn hue and many are already falling especially the horse chestnuts which have already been decimated by leaf miners. These insects live between the upper and lower surfaces of the leaf eating them from the inside. When the leaves fall they overwinter ready to reinfect the new leaves in spring. The multiplicity of different leaf sizes and shapes should give us all pause for thought. Why don’t all leaves look the same?
Scientists have long known about the factors that limit leaf size. Leaves exist to capture sunlight for photosynthesis, so the larger the surface area, the more sun light can be captured. However, the size of the leaf cannot exceed the amount of available water. The evaporation of water from the leaves cools the leaves as well as drawing more water up from the roots to
replace it. Thus a leaf which outgrows the availability of water will soon scorch in strong sunshine. In this way any amount of water creates an impetus towards growth whilst the availability and strength of sunlight restricts growth. Plants find a happy medium between the two according to their own specific set of circumstances.
Yet recently, scientists working in Australia have discovered another variable. Cold. Plants may be just as wary of catching a chill as of catching too many hours of sunshine. The more likely a plant is to experience freezing temperatures the more it will reduce the size of its leaves.
But still leaves have different shapes as well as different sizes, something that can only be accounted for through myriad of tiny adaptations over the enormity of evolutionary time. Each small adaptation is passed down in genetic form from parent to progeny. The evolutionary and ecological history of a plant species is written in the shape and size of the leaf. Style also mirrors function when it comes to plants. They don’t dress up just for our benefit. Getting the style of leaf right for a specific environment can be a matter of life and death. Large round leaves gain more sunlight whilst leaves with sharper edges and rough textures are able to to shade themselves from the strongest rays preventing them from becoming scorched. Large leaves, like those of a banana grow where cold is not a problem whilst small tight leaves such as bundles of pine leaves keep out the cold.
An individual plant may even go so far as to change the characteristics of the leaf according to its own circumstances, how much shade falls on it or what the atmosphere around it is like. Plants are beset by many ecological challenges. Their response over time has been ingenious and hopefully in an era of man made climate change this will prove their salvation and ours.