A tree can be thought of as an array of solar panels on stalks (Colinvaux 1993). Using this analogy, he challenges us to consider the way in which plants are actually constructed (multiple repeti-tions of small leaves on twigs) in comparison with a mechanical ideal:
a large flat solar collector (Figure3.3). Following an analysis of leaf size and shape offered by Horn (1971), Colinvaux argues that the primary advantage of many small leaves is as follows. In the mono-layer designs one huge leaf traps all incident sunlight. But in bright sunlight, photosynthesis would rapidly become saturated. In con-trast, if the panel were subdivided so that the upper layers shaded the lower layers, then the lower layers would receive diffuse light.
A three-layered device (shown in Figure3.3) would outperform a single-layer device, and, of course, plant canopies usually consist of many more than three layers. ‘‘The payoff is increased photosynthesis in bright light’’ (p. 46). In such a design, the shape of the upper panels will affect the kind of shade cast upon lower panels. Irregular shapes reduce effective diameter. This may explain why so many trees have deeply notched leaves: notches increase light transmission to lower
layers of leaves. Such ideas suggest testable hypotheses; for example, upperstorey trees should have leaves arranged in multiple layers, whereas understorey trees should have leaves arranged in a monolayer.
An ideal mechanical model, such as that in Figure3.3(a), certainly stimulates thought. However, precisely because reality is contrasted
Table 3.4. Light response of net photosynthesis of single leaves, under conditions of ambient CO2 and optimal temperature (from Larcher2003).
Plant group
C4plants 20–50 >1500
C3desert plants >1500
Agricultural C3plants 20–40 1000–1500
Heliophytes 20–40 1000–1500
Shade leaves ca. 10 200–300
Young plants 2–5 50–150
Deciduous broadleaved trees
Sun leaves 20–50 (100) 600–>800
Shade leaves 10–15 200–500
Sunny habitats ca. 50 400–600 (800)
Shade ferns 1–5 50–150
Submersed vascular plants 8–20 (30) (60) 100–200 (400)
with such an ideal model, there are many other hypotheses that might be equally tenable. Several come to mind; you may be able to think of others:
1. First is the issue of redundancy. In case (a) (one large layer), damage from a storm might disrupt photosynthesis in a signifi-cant portion of the leaf. In case (b) (many smaller leaves), those that are damaged are unlikely to influence those that remain intact. Think of it this way. If a brick is thrown at a window, which will be more damaged: a window with one large sheet of plate glass, or a window with 100 independent sub-panes? Almost certainly the former, and this may be why sessile organisms, such as plants and corals, that cannot move to safety are constructed on a modular basis. Indeed, Harper (1977) has challenged botanists to think of plants as colonies of independent meristems. Given that nature is full of forces that will damage leaves (wind, rain, hail, herbivores, falling branches) there is likely strong selection against investing too much tissue in any large leaf. Thus canopy trees that are most exposed to storm and wind shear will have multiple small leaves, whereas more protected understorey trees can have larger leaves (a monolayer).
2. Perhaps it works this way. In one large monolayer the surface area is minimal. As the solar panel is subdivided into progressively smaller units, the surface area increases. Since CO2uptake is likely a function of surface area, selection for many subdivided units would enhance CO2uptake.
3. Nature is not perfect. Evolution by natural selection does not produce the best of all possible worlds (although it is frequently misunderstood in this way), it merely tends to select the best of available options. There are, at very least, strict biomechanical limits on the shape and size that plants can take. It is unlikely that any evolutionary pressure would be capable of producing,
1/2 unit P 1/2 unit P 1/2 unit P
(b) m ultila y er (a) monola y er
1 unit P Figure 3:3 The tree on the left has a single huge leaf, whereas the one on the right has multiple leaves with multiple layers. What are the relative photosynthetic efficiencies of these two growth forms? Does this explain why most trees have many small leaves arrayed in multiple layers? (after Colinvaux 1993).
say, large diameter trunks at the ends of small twigs. Similarly, leaves are produced by individual meristems. It is possible that the monolayer in Figure 3.3(a) could be produced by reducing the number of meristems and increasing the amount of leaf tissue produced by each meristem, the limiting case being a single large leaf on one stalk (a structure we might think we can see in leaves of genera such as Podophyllum or Nymphaea). But if branches and twigs are lost in this process, greatly strengthened veins within the leaf must replace them. In the case of Podophyllum, the leaf requires reinforced veins; in Nymphaea the round leaf floats on water. Even in these cases, there is not really a single monolayer leaf; while each shoot may appear to be a single solar panel, these shoots are attached to one another underground by way of rhi-zomes. Thus there are likely many biomechanical restraints upon the form of plants. Natural selection does not produce optimal shapes and sizes; it merely tends to perpetuate the best available options under the multiple constraints faced by many living organisms.
The issue of leaf size and shape will arise in several other con-texts, including the relationship between leaf shape and climate (Figures 4.10, 4.32) and the effect of leaf shape upon early-spring photosynthesis (Figure4.34). If we are as yet unable to answer simple questions concerning why leaves are the shapes and sizes they are, it is no wonder that answers to more complex questions continue to elude us.