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KENNETH V. THIMANN AND GEORGE M. CURRY
the same thing for plants as Loeb for animals, but with greater flair for
physical measurements. Loeb's aim seems to have been mainly to show
that "phototropism" (phototaxis in most cases) is subject to rather simple physical laws, a view which now appears to represent an oversimplification of the matter. The modern biophysical and biochemical
approach has brought out the true complexities of the light-directed
movements, and in the case of plants has led to some understanding
of the intimate relation between phototropism and the control of growth.
II. ^Phototropism" (the Phototactic Response) in Animals
Much of the work on phototaxis, especially on animals, was done in
the early part of this century and has little or no biochemical content.
The methods and approaches used were sometimes those of behavioral
psychology and more often of simple nature study. For this reason no
historical review will be given but the reader is referred to the books
of Loeb (5) and Rose (6); the latter contains a series of extensive
bibliographies of the older literature. Characteristic of the work is the
large number of different animals employed and the scarcity of prolonged analytical studies with any single object selected as specially
favorable. Another characteristic is the frequent lack of precise definition of the light stimulus, either in duration, intensity, or wavelength
(although it was often defined in direction). Effects of temperature and
humidity may often have influenced the experiments also, when bright
lights were used for long exposures, especially on insects. In many instances two separate phenomena are involved, namely, orientation of
the body and actual directed movement. The orientations in insects
have been extensively studied (e.g., by Garrey (7)). In rotifers, orientation is determined by the light-detecting organs, while the photosensitivity of the skin determines movement (8). This participation of two
processes makes the work still harder to evaluate, because of the numerous muscles involved in orientation and locomotion, the participation
of the nervous system, and the role of adaptation and learning.
Nevertheless the work on animal phototaxis did bring out four important characteristics of the reaction, which have a wide (though not
universal) generality and have some very close parallels in the phototropisms of plants.
(1) Reversal of sign. A large number of animals exhibit positive
phototaxis, i.e., movement towards the light source, at low light intensities, but this changes at higher light intensities to negative phototaxis, i.e., movement away. Well-studied examples include the earthworm AUolobophora, the annelid worms Serpula and Spirographis, the
nauplii of the barnacle Balanus perforatus, and a number of insects. The
KENNETH V. THIMANN AND GEORGE M. CURRY
the same thing for plants as Loeb for animals, but with greater flair for
physical measurements. Loeb's aim seems to have been mainly to show
that "phototropism" (phototaxis in most cases) is subject to rather simple physical laws, a view which now appears to represent an oversimplification of the matter. The modern biophysical and biochemical
approach has brought out the true complexities of the light-directed
movements, and in the case of plants has led to some understanding
of the intimate relation between phototropism and the control of growth.
II. ^Phototropism" (the Phototactic Response) in Animals
Much of the work on phototaxis, especially on animals, was done in
the early part of this century and has little or no biochemical content.
The methods and approaches used were sometimes those of behavioral
psychology and more often of simple nature study. For this reason no
historical review will be given but the reader is referred to the books
of Loeb (5) and Rose (6); the latter contains a series of extensive
bibliographies of the older literature. Characteristic of the work is the
large number of different animals employed and the scarcity of prolonged analytical studies with any single object selected as specially
favorable. Another characteristic is the frequent lack of precise definition of the light stimulus, either in duration, intensity, or wavelength
(although it was often defined in direction). Effects of temperature and
humidity may often have influenced the experiments also, when bright
lights were used for long exposures, especially on insects. In many instances two separate phenomena are involved, namely, orientation of
the body and actual directed movement. The orientations in insects
have been extensively studied (e.g., by Garrey (7)). In rotifers, orientation is determined by the light-detecting organs, while the photosensitivity of the skin determines movement (8). This participation of two
processes makes the work still harder to evaluate, because of the numerous muscles involved in orientation and locomotion, the participation
of the nervous system, and the role of adaptation and learning.
Nevertheless the work on animal phototaxis did bring out four important characteristics of the reaction, which have a wide (though not
universal) generality and have some very close parallels in the phototropisms of plants.
(1) Reversal of sign. A large number of animals exhibit positive
phototaxis, i.e., movement towards the light source, at low light intensities, but this changes at higher light intensities to negative phototaxis, i.e., movement away. Well-studied examples include the earthworm AUolobophora, the annelid worms Serpula and Spirographis, the
nauplii of the barnacle Balanus perforatus, and a number of insects. The