![]() ![]() But why is it important to maintain an increased stem microclimate? Hairs on the stem trap a layer of air acting as insulation. Pubescent (hairy) stems, were also recorded as having a higher temperature. This large temperature difference could go some way to suggest a high metabolic rate, high growth and also high tolerance – often seen in weed species. In Magadan, far northeastern Russia, studies on oat, dandelion, thistle and sneezewort plants displayed the same trend of hot stems, from 1˚C to 7˚C, depending on species, location, weather and other factors. This lumen allows the plant to maintain a distinct internal microclimate. ![]() Slicing plant stems open reveals the hollow lumen inside. This certainly reinforces the theory that there is a distinct microclimate within the stem which changes dependent on abiotic and biotic conditions. However, lumen temperature normally dropped slightly below ambient when shaded. In particular, the hollow stem of a dandelion was up to 8☌ higher than the ambient air in sunlight. Generally, in the presence of sunlight, the temperature in the lumen was above that of the ambient air. Temperature probes were inserted into the lumen and results compared with the ambient air and presence of light. Over half of the plant species collected had hollow stems. Of these, there was a large variety in lumen diameter, stem wall width, and stem wall texture. Primarily, over half of the plant species collected had hollow stems, even discounting those with a pith-filled lumen. To start answering these questions, Dr Kevan and his team began collecting and examining plants from across temperate regions including Canada and the UK. For example, a translucent stem may allow more energy through however, it may also mean much is lost – a weak stem tends to droop, so a compromise is required. It’s clear that by understanding these physical properties, Dr Kevan will be able to decipher why particular traits of a plant stem have evolved for certain climactic conditions. Some energy also becomes absorbed into the liquid transport system, and is used up during photosynthesis and plant metabolism (growth). The lumen also loses energy through emissivity to the outside air. This, however, is one of the first times it has been considered with respect to the stem and stem growth. Previously this has been studied with reference to flowers – an increase in temperature encourages growth of the sexual organs, which in turn helps the flower develop quickly, and have a more prominent presentation for enhanced attraction of pollinators. This entire process has been referred to as the ‘microgreenhouse effect’ as it is an exact replica of this on a tiny scale inside the plant stem. This is what maintains the temperature of hollow stems through energy absorption into the ambient gases in the lumen – these circulate through convection, conduction, and re-radiation. When this hits the stem wall, some is reflected, some absorbed into the stem tissue itself, and some transmitted through the stem wall into the lumen. The temperature inside the lumen (hollow centre) of a stem is dependent on the amount of solar radiation (energy from the sun). Dr Kevan wanted to examine a variety of different plant species with different hollow stem traits to understand the relationship between climate and plant stem physiology. There are numerous factors that can affect this, both biotic (with respect to the plant), and abiotic (the external conditions). Factors that normally contribute to air temperature on a large scale are simply applied to the inside of a hollow stem. This rather intriguing study, also known as micrometeorology, is literally as the name describes – studying ‘weather’ on a miniature scale. In order to fill this research gap, Dr Peter Kevan from the University of Guelph is studying differences in plant stem type, and how effectively they regulate temperature. To this day, the physiology of the inside of stems has rarely been examined with respect to heat transfer, a process normally associated with the leaves and flowers. The plant stem is a complex structure, providing a transport system, mechanical support, and also the freshly explored function of thermal regulation. Dr Peter Kevan from the University of Guelph is one of the first in the field to pursue an understanding of this mechanism, in the hope that it will provide economic, environmental, and horticultural benefits to both nature and society. In fact, the reality has much more to do with temperature regulation. A common misconception would be to believe it is solely for nutrient or water transport. Have you ever questioned the reasoning behind hollow stems in some plants? When we think about it there are a surprising number of plants with this feature.
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