Why are ivy leaves grown in the shade, larger in area but lighter in terms of weight than leaves grown in direct sunlight? I am doing a study into the relative size of different ivy leaves from different amounts of sun and shade.

It is strange that you have chosen Ivy leaves for your study. Ivy (Hedera helix) shows enormous variability in its leaves. There are pictures of this on our website. There are differences between the leaves of ground creeping and aerial shoots, flowering and non-flowering shoots and juvenile and mature shoots. How far differences relate to sun and shade is not clear and it may prove difficult to isolate the variables.

However, the observation which you have made on Ivy has also been observed in many other species e.g. brambles, nettles.

Debbie Eldridge comments:- Populations of Brachypodium from shaded populations (non hairy edged morphs) have inherently higher SLA's (SLA = leaf area/leaf dry mass) than unshaded. This can be explained by greater leaf expansion. It is generally thought by many authors that there is fewer layers of palisade cells. What was interesting in my work was that the Leaf Area Ratio (leaf area to total plant dry mass) was not greater in the shaded populations as they channelled a lot of effort into stem growth (much taller than the unshaded) and reproduction. However, SLA (leaf area/leaf dry mass) was consistently higher. The leaves were also arranged along a taller stem which would minimise self shading.
Packham and Willis found the SLA of Oxalis increased as shading increased and Clough et al. found the same in Solanum dulcamara.

Charles Hill comments:- In the summer I did a simple experiment with Y8 measuring nettle leaves and those in the shade were almost twice the area as those in a sunnier spot. Out of interest we did some crude leaf peels with sellotape and found a higher density of stinging hairs on the shade leaves. This is hardly rigorous research but it does suggest the larger area is consistent with cell enlargement which would require a bigger vacuole (mainly water) than increased cell division leading to more cells.

Anne Bebbington has produced a comprehensive review of the subject:

Plants growing in shade often show morphological and physiological differences compared with plants of the same species growing in full sunlight.

The table shows differences which have been found between sun and shade plants.

This table is based on my observations and the table in Adds, Larkcom and Miller The organism and the environment Nelson 1997 ISBN 0174482744

• The large leaves of the shade shoot provide a larger area for trapping light energy for photosynthesis in a place where light levels are low.
• Plants subjected to low light intensity often grow rapidly producing long internodes (the part of the stem between each leaf). Rapid growth may help the shoot to reach light. Pupils can relate this to work they may have done comparing the growth of plants/seedlings in the light and dark.
• The small leaves of the sun plants will provide less surface area for the loss of water through transpiration. Evaporation rates will be high where leaves are exposed directly to the sun
• Various things may cause the colour difference in the leaves e.g. sun leaves may have a thicker cuticle and several layers of palisade cells with the chloroplasts concentrated in them. 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.

In carrying out a number of A level fieldwork projects we have found that:

Dog’s mercury, stinging nettle and bramble all show clear differences in at least some of the above characteristics. Leaves on the outside and inside of the canopies of trees such as beech, lime, plane, elder and hazel also showed differences. I suspect most deciduous trees with a dense enough canopy may show some differences. Working with single trees or rhizomatous plants such as the nettle and dog’s mercury allows you to separate environmentally determined differences from those which have been genetically inherited.

We concentrated in the main on those characteristics which could be readily measured in the field e.g.
Leaf area, internode length and wilting time are all fairly easy to measure.
Leaf thickness can be measured using microcalipers. Broad differences in leaf colour can be recorded using specially devised colour charts.

If facilities are available chromatography may reveal differences in pigmentation. I am also sure that there are also differences in internal anatomy. Differences in the amount of supporting tissues such as collenchyma and lignified cells could probably be seen in fairly crude hand sections with the help of some staining. This would tie in with Barry’s ideas and would be very interesting to look at. Obviously thin sections would reveal any other anatomical differences.

I wonder whether there are differences in the rate of carbon dioxide uptake in sun and shade leaves.

Barry Meatyard was interested in the investigations which could be done to follow up your observations:- Could it be that the leaves nearer the 'outside' get more buffeting by wind / rain etc and produce more support tissue in response? There's a whole heap of investigations that could be done here - for example looking at the area of lignified tissue in the petioles, midribs and veins, measuring the thickness of the leaf etc. The key thing to find out is the source of the density differences - is the dry mass difference the same as the fresh mass difference I wonder?

Other resources:-
There is an exercise with Teacher and Student resources on the Field Studies Council website.

How is the long stem of the stinging nettle an useful adaptation?

It could be said that the stem of any plant is designed to support the leaves so that they can gather the maximum amount of sunlight.

In the case of nettles, the situation is more interesting.

You should read a discussion about plants grown in sun and shade above.

You will find reference there to the longer internodes of plants grown in shade as well as discovering that nettles are especially adaptable to growing in shade conditions.