A modern reexamination of Engelmann's Spirogyra action spectrum would be
very useful. It seems that Engelmann had no way to assure that the PAR
in micromoles per square meter per second was the same for each color,
which could have thrown off the results.
The following passage is from
Darwin, Francis and Acton, E. Hamilton. 1894. Practical Physiology of
Plants. London: Cambridge.
(53) Engelmann's bacterial method.
This depends on the extreme sensitiveness of certain bacteria to the
presence or absence of free oxygen. One of the difficulties connected
with the experiment is the providing a sufficiently sensitive
bacterium. Pfeffer recommends that a pea having been killed by boiling
shall be allowed to purtrefy in 200 c.c. of water; according to Detmer a
pure culture should be made of the bacteria so obtained.
It is best to begin with a study of the behaviour of bacteria mounted
simply under a cover slip. They will be found to swarm round any air
bubbles which may be included in the fluid under the cover slip; and to
collect round the edges of the preparation, and in fact to seek out
sources of free oxygen. If the preparations are sealed by a coating of
olive oil painted round the edge of the cover slip, the bacteria
ultimately become sluggish and come to rest. It is of this fact that
Engelmann's method takes advantage. If a filament of Spirogyra or the
leaf of a submerged plant be included with the sealed bacteria we have
it in our power, by the exposure of the preparation to light, - to
produce free oxygen. Thus all that is necessary is to place the
preparation in the dark until the bacteria are at rest, then to expose
it to light, and to watch the swarming of the bacteria round the green
plant.
By means of Engelmann's Micro-spectral Objective it is possible to cast
a spectrum on the filament of Spirogyra and to observe the distribution
of the swarming bacteria in the different colours. We do not propose to
enter into Engelmann's method of "successive observations" for which
students may consult Engelmann's papers in the _Botanische Zeitung_
from 1881 onwards.
------
The methods book by Detmer refered to above probably has more details on
Engelmann's experiment.
Detmer, W. 1898. Practical Plant Physiology. Translated by S.A. Moor.
New York: Macmillan.
David R. Hershey
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