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Sun leaves and shade leaves respond quite differently to their environment (Goulet and Bellefleur 1986). Sun leaves, relative to shade leaves, are thicker, heavier (per unit area), smaller, lighter colour tone (lighter green colour for example) and have deeper sinuses. Light seems to play a major role in determining such characteristics and whether a certain leaf is bound to have a sun leaf morphology or a shade leaf morphology (Goulet and Bellefleur 1986). One problem that arises from such explanation is that can one observe such bold differences in leaf morphology in other species, or more specifically, are these morphological differences universal to all tree.
These morphological differences should definitely be universal; one must be able to observe the characteristics of sun leaves and shade leaves in essential all kinds of trees that have leaves. One way to observe these unique characteristics of leaves is to go out and collect many leaves of both sun characteristics and shade characteristics. Then do a more thorough evaluation of these leaves and quite possibly one will surely observe many differences between sun and shade leaves.
If sun leaves and shades leaves do in fact have many differences then sun leaves should weight more than shade leaves because sun leaves are thicker having more photosynthetic cells. Sun leaves also have a thicker waxy cuticle and therefore must weight more than shade leaves in general (Olaveson and Rush 2010).
If these differences do exist then shade leaves should, in general, be larger because sun leaves will have more indentation to let more light pass through to the shade leaves. Also this will ultimately help sun leaves to dissipate heat since sun leaves are in direct sunlight because due to the more indentation, sun leaves will have a higher perimeter to surface area ratio (Olaveson and Rush 2010).
Materials and Method
Data presented in this report was obtained from forty sun leaves and forty shade leaves of a Norway maple tree (Acer platanoides L). In late September, an Acer platanoides L located on University of Toronto at Scarborough (UTSC) Science Wing patio was chosen for this observational study. The tree size, notable features surrounding the tree and obvious leaf canopy were observed. Its leaves were examined in the lab at UTSC. General observations of the leaves were done such as its shape, colour, texture and overall condition. A more quantitative analyse of the leaf weight, length, width, perimeter, surface area and sinus area were also done in the lab. Leaf weight was measured on a weighing scale, length and width were measured by an aid of a ruler, the perimeter was measured by an aid of a string that was outlined around the leaf and surface area and sinus area were measured by tracing the leaf on a 1 cm2 graph paper. Using the quantitative data, the shape ratio (weight / surface area) and specific leaf weight (sinus area / surface area) was calculated for each leaf (Olaveson and Rush 2010). The data were recorded and statistically analyzed using Microsoft Excel.
An interesting aspect of the study were the trends in leaf size. The length and width of shade leaves are significantly greater than of sun leaves (Table 1). This directly relates to the surface area of the leaves; as such, shade leaves also have a greater surface area then sun leaves on average (Figure 1). The overall trend with regards to the relative size of the leaves is that length, width and surface area of shade leaves are greater then of sun leaves (Table 2).
Related of the leaf size, shade leaves also tend to weight more then sun leaves on average (Figure 2). In fact, there is a significant difference in the mean weight of the two types of leaves (Table 2). So the trend for leaf weight is that as the light intensity decreases, the leaf weight increases accordingly; thus shade leaves weight more.
The perimeter of the leaves in this study did not provide any significant results (Table 1). Although is there is a difference in the mean perimeter of sun and shade leaves, it ultimately leads to no unique trend because this difference is very tiny. Nonetheless, there is a difference so one cannot ignore it nor can one make any conclusions based on it; thus it provides no concrete support in this study.
The results from the statistical test in table 1 and table 2 clearly show that indeed length, width, surface area and weight of the sun and shade leaves do in fact differ quite significantly; on the contrary it shows that mean perimeter of the sun and shade leaves have no significant difference.
Mean leaf shape ratio of sun leaves is greater than of shade leaves. The same can be said for the maximum and the minimum values of the shape ratios of both sun and shade leaves. So generally sun leaves have higher shape ratio (Table 3). Another quantity that was calculated in this study was specific leaf weight (SLW). On average SLW for shade leaves was higher than sun leaves; this makes sense because as mentioned earlier, shade leaves generally have a higher weight and surface area as compared to sun leaves. So the general trend outlined in table 3 is that sun leaves have a higher shape ratio while shade leaves have a higher SLW.
The length, width and surface area of shade leaves were found to be greater than sun leaves. The same trend was observed with regards to the weight; shade leaves consistently weighted more than sun leaves. So the trend that as the light intensity decreases, the leaf weight and relative size increases makes perfect sense; because if a leaf is getting less light it will want to make full use of that light and thus it must have a larger surface area to absorb as much light as possible. Although the results for leaf length and width complies with our prediction, the result for leaf weight however do not comply with our prediction. These results also seem to reflect the results from other similar studies (Goulet and Bellefleur 1986). So clearly light intensity is the main cause for this phenomenon because essentially that's the only condition that changes among sun and shade leaves due to their position on the tree (Lei and Lechowicz 1998; Ashton et al. 1999).
We encountered one problem in this study: the insignificant results from the mean perimeter of the leaves. As mentioned earlier it provided no significant conclusions. So a better way to replicate this study would be to completely eliminate perimeter and instead replace it with leaf colour intensity. That will provide a better representation of the differences in sun and shade leaves.
Mean leaf shape ratio seemed to indicate that as light intensity increases so does the leaf shape ratio. Again, this complies with our prediction made earlier that sun leaves have deeper sinuses; and thus more light is transferred to the shade leaves due to this and in turn shade leaves have shallow sinuses so more light is absorbed (Beaudet and Messier 1998).
Another quantity that we calculated in this study was SLW. On average SLW for shade leaves was found to be higher than sun leaves; this in indicative of the leaf thickness and we did indeed find that shade leaves are thicker in comparison to sun leaves (Olaveson and Rush 2010).
In light of the results presented in this study, our research hypostasis is clearly support by our finding. Indeed there are many key differences in sun and shade leaves which result in the categorization of such leaves as sun or shade leaves based on its morphological characteristics.