Comparing sun and shade
Several common species of plants show clear morphological differences between leaves which grow in full sunlight and leaves which grow in shade.
In this investigation, students sample sun and shade shoots of stinging nettle and consider differences in leaf size and internode length.
Through this activity pupils will
- use systematic sampling to compare and contrast two morphological differences within a single species of plant
- consider the results of their investigation in relation to photosynthesis and the adaptations of plants to intraspecific competition.
Resources to download
Background information for teachers
It is sometimes difficult to separate out all the biotic and abiotic variables which affect the characteristics of plants. By concentrating on one species in two slightly contrasting habitats, it is possible to collect scientific data to consider the effect of light levels on shoots and leaves. Since nettles spread vegetatively, a large clump of genetically-identical shoots can straddle different light levels.
Leaves which grow in the shade (‘shade leaves’) are generally larger in area but thinner than leaves which grow in full sunlight (‘sun leaves’). Sun leaves become thicker than shade leaves because they have a thicker cuticle and longer palisade cells, and sometimes several layers of palisade cells. The larger shade leaves provide a larger area for absorbing light energy for photosynthesis in a place where light levels are low. In contrast, smaller sun leaves will provide less surface area for the loss of water through transpiration. Transpiration rates will, of course, be higher where leaves are exposed directly to the sun.
Shoots grow more quickly in height where light levels are low. This rapid growth helps the shoot to reach light. The length of the internode (the part of the stem between each leaf) is longer for shade shoots than sun shoots.
A final variable which is less straightforward to measure in the field is leaf colour. Sun leaves tend to be a lighter green than shade leaves and they may also be tinged in red. Shade leaves generally contain a greater mass of chlorophyll and are darker green in colour. In shade leaves, the chloroplasts move within the cells to take up a position where they will absorb the maximum light without shading other chloroplasts below them. The chloroplasts are evenly distributed between the palisade and spongy mesophyll layers. By contrast, in sun leaves, the chloroplasts take turns in the bright light and then shelter in the shade of others whilst they make use of the light they have absorbed. Too much bright light would destroy the chlorophyll. In sun leaves, most of the chloroplasts are found in the palisade layer. There may also be a difference in the amounts of different pigments in the leaf. Anthocyanin pigments are produced in the stems and leaves of the sun shoots. These red pigments help to protect the chlorophyll from excess ultra-violet radiation.
Most of these adaptations take place during leaf development - there is little a leaf can do if its light conditions change. Shade leaves can be up to five times more efficient in harvesting the same amount of sunlight as sun leaves. But shade leaves lose water by transpiration quicker than sun leaves given the same temperature and humidity conditions.
Other points for discussion
Some surveys have shown that the density of stinging hairs on nettles is higher on shade leaves. Wilting time also varies; leaves on cut sun shoots take much longer to wilt than leaves on cut shade shoots, probably because sun leaves are more effective at limiting transpiration.
Make sure that the pupils sampling the nettles are wearing rubber gloves and that their arms and legs are covered.