Plants Growing at Water's Edge

Have you ever wondered why plants and trees growing at the margins of pools and lakes of water look magically beautiful? They grow branches and leaves towards the water. They grow along and across the water surface but never touch it. How is that even possible? Let's look at the processes involved in plants to produce this eye-catching scenery.

Tree Branches Over Water

It is obvious that plant roots grow towards water because of positive hydrotropism. Root caps at the ends or roots are responsible for this type of growth movement. This makes sense because the root system is responsible for the absorption of water and minerals. But why does the branches and leaves of a plant at the edge of a pool of water grow towards it?

More Light Favors More Growth

Stem and its parts exhibit positive phototropism and negative gravitropism. That is to say the stem grows towards light and in the opposite direction of gravity. Trees in the middle of land receive sunlight from the sky and surrounding. However, the amount of light it gets is not the maximum because of neighboring trees and geographical features. When it comes near water, the tree is exposed to more light from the site of water as 

1. There are lesser obstacles to light 
2. Water reflects more light than the land

This results in a stem preferring to grow towards water. 

 Growth Limited at the Water Surface

Branches and leaves may grow towards water, seemingly neglecting negative gravitropism. But they don't touch the surface of water! How do the plants know when to stop growing branches to avoid touching the water or crossing into the water? What mysterious instincts do plant have? Apparently, this phenomenon can be explained in terms of transpiration. Transpiration is the process by which water moves from the roots to the leaves where it's evaporated to the atmosphere.

How Transpiration Affects Plant Growth

Rate of transpiration can be correlated with the rate of growth of a plant. Plant growth requires nutrients and minerals. Transpiration helps uptake and transport of minerals in the plant. So, regions with slower transpiration gets lesser amount of minerals. This can result in slower growth in such parts of the plant.

Tree transpiration was not correlated with stem diameter, but it was closely related to a local leaf area competition index.

Source: Bréda, N., A. Granier, and G. Aussenac. "Effects of thinning on soil and tree water relations, transpiration and growth in an oak forest (Quercus petraea (Matt.) Liebl.)." Tree physiology 15.5 (1995): 295-306. 

However, there wasn't sufficient quantitative evidence to support a relationship between rates of transpiration and growth of a plant.

Transpiration and Humidity

Rate of transpiration is affected by several factors. Transpiration decreases with low temperatures and high relative humidity. This can be observed over a water source.

When water is left out open to air, some of it takes up heat and becomes water vapor. Distribution of vapor gives rise to a gradient of relative humidity above the surface of water. Therefore, the possible rates of transpiration are reduced as it comes closer to the surface of water; resulting in slowing down of growth. This stops growth of leaves and branches through the water and gives rise to a nicely arranged layer of canopy of leaves over the water.

Shedding and Decay

Plant parts that do touch the surface are at more risk of decay and rotting because of favorable conditions for bacteria and fungi. Therefore, such leaves are shed or lost from the branches allowing the leaves just above the water to survive