The first exposure to scientific experimentation for many a precocious youngster is putting a bean seedling in a box, poking a hole to let some light in, and watching the seedling grow toward the light even after it is turned around. The study of this phenomenon, called phototropism, has a long history — but questions remain.

An article in Current Biology on phototropism (plants’ attraction to light) reminds readers that Charles Darwin wrote more than The Origin of Species and The Descent of Man.

In his seminal publication The Power of Movement in Plants, Darwin used mostly monocotyledonous grass seedlings to study phototropism. These simple but effective experiments still cement our current understanding of how economically important plants such as oat, rice and maize are established upon their emergence from the soil. Darwin’s work inspired generations of plant physiologists to investigate the phototropic response of flowering plants. Subsequent theories were introduced to explain how light brings about this growth adaptation. One notable concept is the Cholodny-Went hypothesis, which states that light from one direction drives lateral movement of the phytohormone auxin from the irradiated to the shaded side of the stem. It is this greater accumulation of auxin on the shaded side that stimulates the differential growth of the seedling towards light (Figure 1). Although central to the Cholodny-Went hypothesis, a complete cellular assessment of these lateral auxin movements and how they are regulated is still lacking. However, much progress has been made in the last decade or so in defining key signalling events associated with phototropism using the dicotyledonous genetic model Arabidopsis thaliana. In this Minireview, we summarize recent advances in uncovering the molecular complexity underlying this emblematic plant growth phenomenon. [Emphasis added.]

So here we have a puzzle in biology that Darwin left unfinished since 1880 when his book The Power of Movement in Plants was published. Progress has been made, but recent advances show more of the “molecular complexity” of the phenomenon. Which theory — intelligent design or unguided natural selection — best explains it?

According to The Darwin Correspondence Project, the book “was intended to answer a specific set of objections to his theory of evolution by natural selection.”

This 1880 book sought to illustrate that evolution could account for changes in behavioral responses. In the conclusion of the book Darwin argues that gradual modifications in the development of plants in response to natural forces such as water and light could account for all botanical adaptations.

The operative words here are “natural forces… could account for… all… adaptations” — all of the botanical wonders of the world. That’s a tall order. Does it include flowers, seed dispersal, and everything down to the molecular machines in plant organelles? Does it include plants’ adherence to the Golden Ratio? Of course it must. Had Darwin left anything out, natural selection would prove itself inadequate. Teleology might sneak back in.

Unquestionably, Darwin’s attempts to thoroughly naturalize plant behavioral responses “inspired generations” of other investigators. The appearance of empirical investigation is appealing to the scientific mind: controlled experimentation, appeal to natural forces, testing hypotheses. The right question, though, is not about forces and methods. It should be whether naturalism is true. Do “natural forces… account for all botanical adaptations”?

Fast forward to the present. The Current Biology paper says very little about evolution, and nothing about natural selection. The first paragraph mentions evolution only in very general terms before recounting the story of Darwin’s inspiring book (quoted earlier):

Plants are sedentary organisms that depend on sunlight for photosynthesis. Consequently, they have evolved the ability to alter their growth to optimise light capture and increase photosynthetic productivity. Phototropism is a classic example of such an adaptive growth response.

We see this “argument by assertion” frequently in papers. Darwinists say this-or-that “has evolved,” then spend the bulk of their time describing the operation of the phenomenon, not its origin. That’s the case here. After the assertion, details flood in about hormones, molecular machines, receptors, signaling cascades, genes — a vast array of sophisticated structures and their coordinated reactions. At the end of the paper, have “natural forces” accounted for the phenomenon?

Extensive work with Arabidopsis has shed considerable light on the identity of key signaling events associated with hypocotyl phototropism. It is well established that photoactivation of phototropin blue light receptors is necessary to bring about this differential growth response. Yet the mechanisms that couple receptor activation to changes in auxin mobilization remain largely mysterious. Genetic analyses indicate the involvement of multiple auxin transporter proteins in addition to a number of important regulatory components, including D6PKs, PKS proteins, NPH3 and related proteins. Arabidopsis has been instrumental in providing some light at the end of the tunnel in understanding this complex growth phenomenon. However, deciphering how these factors are regulated at the biochemical level and how they are integrated to coordinate light-driven changes in auxin mobilization, pH and microtubule reorientation associated with phototropic growth still remains a major challenge for researchers in the field.

Where has the Darwinian approach provided understanding? It’s been a fool’s errand. It started out being mysterious, and it’s still mysterious. All the investigative activity — admirable as it is — has been a distraction from the real question: can natural forces account for phototropism?

Look at all the implicit design in the passage quoted above. We see (1) signals, (2) receptors, (3) mechanisms, (4) transporters, (5) regulatory components, (6) factors being regulated, (7) integration of factors, (8) coordination, (9) mobilization, (10) reorientation — all in one short summary paragraph. If it weren’t for the botanical jargon, you might think this is talking about a well-run company or army.

To be sure, there are cases in nature of nonliving chemicals reacting to light. Phosphorescent rocks glow after light is shined on them. One abstract in Chemistry of Materials describes certain “photoreactive crystals” that, though rare, can start “popping, hopping, and leaping” when irradiated with UV light. A paper in The Journal of Physical Chemistry even talks about “phototropic liquid crystals.” In these cases, we might well suppose that natural forces can account for the phenomena.

The difference with plant phototropism, though, is profound. None of the photoreactive chemicals have genes, signals, transporters, or all the other things listed above. None of them are integrated with regulatory components. In short, none of them have the same level of information that is (a) complex, (b) specified, and (c) arranged for a function. Phototropic liquid crystals do not grow seeds and bear young. Plants are vast systems of molecular machines that dynamically respond to the environment through signals. Without this machinery and the information that makes it work, shining a light on a plant will make it wilt and die. Plants employ natural forces (light, gravity, etc.) in phototropism, but the forces do not explain its origin. Forces are necessary but not sufficient, in other words, to account for the phenomenon.

In our uniform experience the only “force” or cause that can create functional systems at this level of complexity is intelligence. Intelligence can create information and impress it on matter, making it do things that unguided natural forces cannot (think: airplanes). Intelligence, further, cannot be reduced to the four fundamental forces of nature. So by both negative arguments (the inadequacy of material forces) and positive arguments (our uniformity of experience with complex systems), we can affirm that intelligent agency does “account for” the phenomenon of phototropism. To the Darwinians, after 135 years of trying, it’s still “largely mysterious.”

Image by Tangopaso (Own work) [CC BY-SA 3.0], via Wikimedia Commons.