More on Leaves – the Story Continues!
It may be an overly simplistic statement that we often forget, but we could not survive without plants and their leafy appendages! Regardless of whether the ‘leaf’ is found on a moss, fern, evergreen or a flowering plant, the job of that ‘leaf’ is to convert Carbon Dioxide (CO2) and Water (H2O) into carbohydrates (C6H12O6) and Oxygen (O2), typically in the presence of light. The need for Oxygen is certainly understandable, but we often forget how carbohydrates – the fruit and vegetables on the table, the grasses that feed livestock, the lumber that builds our homes or the ancient planktonic ocean life that created the oil deposits of today to name but a few – are essential for our day-to-day lives.
Despite our dependence upon leaves, they are undoubtedly the most susceptible plant organ to damage whether that be wind, insects, animals and even humans – after all, who has not plucked a leaf from a branch at some point in our lives or simply eaten lettuce or spinach! Beyond surviving physical damage, the biggest daily challenge facing a leaf is the balancing act it plays between absorbing the appropriate amount of sunlight and CO2 while regulating its temperature and water loss. Heat and water are as much of a concern to plants and leaves as they are to people! Compound leaves – those leaves which are divided into leaflets and are connected by rachises – obviously have smaller leaflets that in turn help to keep the leaf cooler. Simple leaves, with their larger mass of foliage exposed to the sun, regulate temperatures through lobbing of the leaf, such as seen on a Japanese Maple or by creating more complex leaf margin configurations in an effort to disperse the absorbed heat more rapidly.
Interestingly, even on the same shrub, tree or large perennial, leaves vary in shape, color and size by the degree of sunlight they receive. Leaves can loosely be divided into ‘Sun’ or ‘Shade Leaves’ based upon whether they are located on the perimeter of the canopy or within the canopy. As seen in the image of 2 Witchhazel leaves on the right, ‘Sun Leaves’ are both smaller and thicker than ‘Shade Leaves’ and are typically lighter green in color. Sun Leaves are also more effective at converting CO2 to O2 per unit area of leaf. By comparison, Shade Leaves are larger, providing more surface area for light absorption, are darker green and have more simple leaf margins.
The difference between Sun and Shade leaves is also manifested within the internal structure of the leaf as well. When studied in cross section, leaves are typically composed of several distinct layers of cells, each of which perform succinctly different functions. See the image on the below (Source: Zephyris on Wikipedia [CC License]). The outer layer on both the upper and lower leaf surface is a waxy cuticle, which helps reduce water loss. The thickness of the cuticle varies depending upon the plant species and the environmental pressures upon that species. The thickness also varies on light levels, with Sun Leaves – with their more exposed location on a plant and greater susceptibility to dehydration – having a thicker cuticle layer than Shade Leaves. Beneath the cuticle is the epidermis and much like the epidermis or skin of humans, the role of this layer is to protect the regions within! One major difference between humans and plants are the openings typically found on the bottom of the leaf called stomata that allow ‘air’ to freely pass into the leaf. Another difference is the transparency of a leaf’s epidermis, allowing sunlight to reach those cells within the leaf. In some Shade Leaves, chloroplasts or the cells that contain chlorophyll and other pigments that conduct photosynthesis, are also located within the epidermis. Between the upper and lower epidermal layer is the mesophyll, which comes from the Greek meaning middle leaf. The mesophyll has two distinctly different layers: the upper Palisade Layer and the lower Spongy Layer. Like the rocky cliffs along the Hudson River of NJ, the palisade layer consists of rows of cylindrical cells which are individually separated by minute air spaces. Palisade initially described a fence composed of rows of vertically aligned posts, which these cells resemble under high magnification! Receiving the greatest proportion of direct sunlight, it only makes sense that these cells would contain a high proportion of chloroplasts. Interestingly, the chloroplasts of ‘Shade Leaves’ contain a higher concentration of chlorophyll, giving them their darker color. The Palisade Cells are thin walled and the chloroplasts actual line the cells just below the cell membrane. In Shade Leaves, the Palisade Layer is only one cell thick, due to the diffuse nature of the sunlight and its inability to penetrate deeply into the leaf. However, in Sun Leaves, the Palisade Layer can be upwards of 5 cells thick! This explains in part why Sun Leaves are smaller and thicker than Shade Leaves, since there are more cells and chloroplasts per unit of leaf exposed to sunlight. The greater overall concentration of chloroplast also explains why sun leaves can convert more CO2 to O2 than Shade Leaves per unit of exposed leaf surface!
The differences in the Palisade layer between Sun and Shade Leaves also explains why most plants go through a period of leaf drop when brought inside for the winter or scorch when moved outside to a sunny location for the summer. When brought inside, the higher concentration of Sun Leaves is not as efficient as Shade Leaves in the lower light levels of a home. In response, the plant drops the Sun Leaves in exchange for the production of Shade Leaves. In contrast, when the plant is brought back outside for the summer, the thinner cuticle and Palisade Layer along with the larger leaf size of the Shade Leaves will cause the leaf to scorch when exposed to periods of direct sunlight!
Located below the Palisade Mesophyll is the Spongy Mesophyll, named for its similarity in appearance to a sponge when studied under magnification. This layer consists of ‘ball-shaped’ cells, which in contrast to the layer above, permits larger intercellular spaces for ‘air’. Due to the proximity of this layer to the stomata – the portals through which air passes – this open matrix of cells allows the leaf to temporarily store CO2 and allow photosynthesis to potentially continue after the stomata close during periods of rapid water loss. Cells in the Spongy Mesophyll can contain chloroplasts, but in far lower levels that the Palisade layer.
Neither type of leaf, Sun nor Shade, is truly superior in form or function – they simply operate in the most efficient manner possible in their given environment. Although leaves appear to most individuals and gardeners as an ornamental attribute of a plant that can be readily damaged, they are in fact very structured organs that a plant can modify to some degree to match the environment! Aren’t plants cool!