Red means stop?

Plants come in more colors than just green. Here is a normal leaf of one of the common understorey fern species in the rainforest.

And here is a different leaf of the same species – but this one is red! Why?

To answer this question, you need to know one more fact: the red leaf is actually a young, not an old leaf. In temperate climates, red leaves tend to be autumnal and reflect leaf senescence. In the tropic, red leaves tend to be associated with new growth. So what’s going on?

There are several hypotheses for the adaptive significance of red coloration. One idea is that the red pigmentation (containing anthocyanins) protects leaves from damage from solar radiation before they develop a full complement of photosynthetic pigments like chlorophyll. Because chlorophyll is expensive to produce, it makes more sense to invest first in a less expensive pigment, then later deploy chlorophyll if the leaf survives to maturity. A second idea is that the anthocyanins inhibit fungal growth on the leaf. A third idea is that anthocyanins are unpalatable to insects. A fourth idea is that the red coloration camouflages the leaves from colorblind herbivores. A fifth idea is that the red coloration serves as a warning to non-colorblind herbivores. Some of these ideas are discussed in the scientific literature (e.g. Dominy et al. 2002) but there is not yet a strong consensus – any or all of these hypotheses could be true, depending on the species. Ultimately, red coloration may be a complex trait with multiple evolutionary drivers. It makes for beautiful walks in the forest.

Many other things in the forest are also interesting colors – insects and fungi too. Sometimes this coloration has a warning function, but not always. Here are two examplees for you to consider – what do you think the coloration is doing?

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