Engelmann's major contribution to the field of physiology emerged from a study lasting from 1873 to 1897, in which he observed the contractions of
striated muscles. Focusing on the visible bands of fibers in the muscles, he noted that the volume of the
anisotropic band increased during contraction, whereas the volume of the
isotropic band decreased. He
theorized that it was this interaction between the two bands which allowed for
muscle contraction. He also demonstrated, after
experiments with dissected
frogs in 1875, that contractions of the
heart were caused by the heart muscle itself, not an external
nerve stimulus, as was previously believed. Engelmann performed three significant experiments involving photosynthesis: In 1881, he observed the movement of bacteria towards the chloroplasts in a strand of
Spirogyra algae. Engelmann hypothesized that the bacteria were moving in response to oxygen generated by the photosynthetically active chloroplasts in the algae. This was one of the first documented observations of
positive aerotaxis in bacteria. In 1882, he performed his famous action spectrum experiment using a device designed and built by
Carl Zeiss. The modified microscope had a prism which could produce a microscopic spectrum on a microscope slide. The device could also distinguish and measure different wavelengths of light making it a “micro-spectroscope.” Engelmann used this device to illuminate a strand of
Cladophora (not
Spirogyra) with light from the
visible spectrum, exposing different sections to different wavelengths (or colors of light). He added the oxygen seeking bacteria B. termo to this setup and noted where they accumulated (Note: Four years later, Hauser concluded that B. termo had been mislabeled and was not one, but three species of bacteria of the genus
Proteus ). Their clumping allowed him to see which regions had the highest concentration of oxygen. He concluded that the most photosynthetically active regions will have the highest concentrations of bacteria. The bacteria accumulated in the regions of red and violet light, showing that these wavelengths of light generated the most photosynthetic activity. However, his experiment was somewhat flawed because he used the sun as his light source. He failed to account for the fact that the sun does not emit all visible wavelengths of light at the same intensity. However, further analysis of plant pigments proved that his results were valid. A year later Engelmann discovered that
purple bacteria utilise
ultraviolet light in the same way. Moreover, he was among the first scientists who traced a relationship between the availability of wavelengths in underwater light (which are changing/decreasing with depth) and the occurrence of those
phototrophs which can efficiently absorb and use them. == Musical significance ==