Acquacheta in HDR
The von Karman vortex street of shed vortices that form the wake of a stationary cylinder are a classic image of fluid dynamics. Here we see a very different wake structure, also made up of vortices shed from a cylindrical body. This wake is formed by two identical cylinders, each rotating at the same rotational rate. Their directions of rotation are such that the cylinder surfaces in between the two cylinders move opposite the flow direction (i.e. top cylinder clockwise, bottom anti-clockwise). This results in a symmetric wake, but the symmetry can easily be broken by shifting the rotation rates out of phase. (Photo credit: S. Kumar and B. Gonzalez)
Attention birders! Can you guess the species in this x-ray image, taken in our Animal Health Lab? Hint: You can see him now, in our Aviary!
Answer: This is a marbled godwit (Limosa fedoa). Godwits can be seen in California coastal areas, predominantly in the fall. The bird wasn’t sick, but the image was taken as an important reference in examining other, healthy birds.
Some fun facts from our own Dr. Mike Murray:
Note the sigmoid or S-shaped neck.
The very faint whitish circle around the place where the eyes should be are the sclera ossicles, a set of bones that are in the bird’s “white of the eye.” Remember you are seeing them for both the left and right eye somewhat superimposed.
The wind pipe (trachea) can be seen extending from the area under the jaw down to where it enters the bird’s body. Looking closely, you see the cartilage rings that make up the trachea. In mammals, they are ‘C’ shaped. In birds, they are complete rings.
The arch-shaped bones at the bottom of the neck are the bird’s clavicles (we call them collarbone in humans). In poultry, they are affectionately referred to as the “wishbone.”
Why trees can’t grow taller than 100 metres
TYPICALLY, the taller the tree, the smaller its leaves. The mathematical explanation for this phenomenon, it turns out, also sets a limit on how tall trees can grow.
Kaare Jensen of Harvard University and Maciej Zwieniecki of the University of California, Davis, compared 1925 tree species, with leaves ranging from a few millimetres to over 1 metre long, and found that leaf size varied most in relatively short trees.
Jensen thinks the explanation lies in the plant’s circulatory system. Sugars produced in leaves diffuse through a network of tube-shaped cells called the phloem. Sugars accelerate as they move, so the bigger the leaves the faster they reach the rest of the plant. But the phloem in stems, branches and the trunk acts as a bottleneck. There comes a point when it becomes a waste of energy for leaves to grow any bigger. Tall trees hit this limit when their leaves are still small, because sugars have to move through so much trunk to get to the roots, creating a bigger bottleneck.
Jensen’s equations describing the relationship show that as trees get taller, unusually large or small leaves both cease to be viable (Physical Review Letters, doi.org/j6n). The range of leaf sizes narrows and at around 100 m tall, the upper limit matches the lower limit. Above that, it seems, trees can’t build a viable leaf. Which could explain why California’s tallest redwoods max out at 115.6 m.
Gemsbok in Namibia by Jess Sara Wright