(Porcu)pine tree needle firing system?

Question

Info

I've created a pine tree that stabs its needles into people. Here's the way I did so:

Using genetic engineering, my fictional scientists inserted keratin-producing genes into a pine tree (gymnosperm, a clade of plants that while fruitless have complex reproductive systems and vascular tissue) embryo. Through some more work, they produced a tree that has a unique trunk.

As the meristem growth causes cork and vascular cambium to form, plant cells near the vascular bundles grow very close to the skin of the tree (near the outer ring of vascular tissue and cork cambium) and begin to elongate due to the plant hormone auxin, which stimulates elongation of cell walls without triggering cell division. As they stretch, they grow bundles of myosin. Then they produce actin filaments to power what is essentially muscle movement. These cells grow into "muscle" fibers and small threads of tissue that, while not as complex as human muscle (which has tissue bundles held together by connective tissues such as epimysium) are capable of exerting significant pressure. They grow underneath the developing quill.

The keratin plates, in the meantime, grow out of the developing muscular bundle and form a quill, like that of a porcupine . . . with two differences: the quills are stiffer, and they are pure keratin, like very fine, sharp claws. (A porcupine pine tree!)

When you go to touch the tree or tap one of the needles now embedded (point facing out) in the trunk, the muscles react to the pressure and force the needle(s) forward.

The quill is finely barbed, since according to Science, barbed quills required approximately half the penetration force of the barbless quills, or 56% of the pressure of a hypodermic needle to breach (human) skin.

From what I was able to gather, the pressure needed for a hypodermic needle to break through skin is 20 kPa (kilopascals). 56% of 20 is 11.2, so taking pressure as P, F as force, and A as area of surface on contact, $$P=\frac{F}{A}$$.

The area of my porcupine quills is exactly 1mm² at the tip. The pressure exerted needs to be sufficient to break through skin, so using this calcuator to reverse-engineer the equation, I need exactly .0112 Newtons of force to generate the pressure needed (for the tip.) That doesn't seem like too much for the muscles in the trunk to push the quill forward. I think that the defense (stabby needles) works, but I'm not sure:

The Question

Reality-check my system, please. Would the defense realistically work, or did I miss something/are my numbers wrong?

Bonus points for: telling me whether I should make the needles sharper, thicker, or thinner/pointier to make the tree more effective.

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Thanks to those in the Sandbox for checking my numbers and providing a little feedback.

Answer 1

Spiny piney tree! I dig it.

This is a really cool idea, and certainly plausible. Keratin has a (relatively) high compressive strength so the needles would be suitably sturdy, the force required is a reasonable one, and the Rule of Cool is definitely in your favor. However, there are still a few points of concern I would have as a reader.

1. Plant cells are rather inflexible.

Plant cells differ from animal cells in several ways, but the most relevant one for our scenario is the presence of the cell wall. This structural layer provides rigidity to the cell and prevents plasmolyzing, but would serve to largely block shape changes in the cell that are required for normal muscle movement.

Muscles in animals have a simple "unit" of action - the sarcomere. This structure is composed of actin and myosin filaments. As the myosin protiens are powered by ATP, they "walk" along the actin filaments and bring the opposite ends of the sarcomere together, causing the cell to contract. In the gif below, the light pink bands are the ends of the sarcomere and you can see them pulled together and apart as the actin and myosin slide past each other.

This mechanism is problematic because plant cells can't squish like this. The thick and sturdy cell wall will prevent the deformation required, and thus any sarcomeres would be applying tension to the cell wall rather than distorting it. This leads to the all-or-nothing kind of scenario, where the cell wall would either sustain the strain or would collapse catastrophically.

2. Your structure seems to have "muscle" cells in parallel, not in series.

You'll note that there are multiple sarcomeres attached end-to-end in the animation above, just as they are in real life. This allows large actions to occur over small time scales - even though molecular motors like myosin move at maximum speeds on the scale of micrometers, lining up 100,000 of them allows our muscles to function much faster.

It sounds like your setup has a "ring" of lengthy cells surrounding the needle that the needle then slides out of. This will simply occur too slowly without the multiplicative power of serial sarcomeres described above. To get an idea of how fast a cell like this could actually move something, I humbly defer you to videos of Bacillaria paradoxa, the famous slidey diatom. These colonial creatures crawl back and forth past each other via myosin attachments on the outside of their silica frustules. The speed is maybe 10 micrometers per second - the frustule here is about 100 um long and it takes maybe 10 seconds to slide the whole way.

An alternative idea

May I recommend a mechanism similar to the cnidocysts of jellyfish? These "stinging cells" could be much more dangerous and might actually minimize the engineering required.

Cnidocysts operate like spiky, inflatable, water balloons. When inactive, they have a calcium imbalance between the interior of the cell and the exterior. When triggered, the osmotic pressure essentially "inflates" the cell, forcing the spike on the front to invert and thrusts it into a triggering stimulus. This seems like it would be a perfect mechanism for your tree. Although it requires a highly flexible cell/structure, trees already use turgor pressure to maintain cell shape, and that high-pressure environment would facilitate ejection, and there's no need to insert animal genes into the plant kingdom. The tip of these nematocysts could still be made of keratin and tipped with a nasty substance to improve fatality rates. If, you know, the spiky water balloon isn't deadly-looking enough: