6. PHOTOTROPISM AND PHOTOTAXIS
249
The relative insensitivity of many animals to the ultraviolet is mainly
due to the powerful absorption of the ultraviolet by the eye lenses,
which act as cut-off filters in this region (15). The data as a whole
suggest the action of several red to purplish light-absorbing systems,
and it is evident that what is being measured is essentially the absorption spectra of a group of "visual purples" (see Chapter 7 by Wald, in
this volume).
III. Phototropism in Plants
So much of the work on phototropism has been in fields remote from
biochemistry that it hardly seems appropriate to present a complete
historical review here. Instead, this presentation will be limited to those
aspects of phototropism which have been treated biochemically or biophysically, or are at least capable of such treatment. An extensive review of early work on phototropism, along with some original material, was presented by du Buy and Nuernbergk in 1932-1935 (16).
Briefer surveys have been given by van Overbeek (17), du Buy (18),
Went and Thimann (19), and Oppenoorth (-20), the last-named author
adding much additional data on the part played by auxin. More recently the subject has been reviewed by Brauner (21), Went (22), and
Galston (23).
A phototropic curvature results from a difference in growth on the
two sides of an elongating organ. If the curvature is positive, i.e.,
towards the light, it could be due to: (a) deceleration of growth on
the lighted side (or a greater deceleration there than on the shaded
side); (b) acceleration of growth on the shaded side (or a greater
acceleration there than on the lighted side); or (c) both. Corresponding
possibilities are offered for negative curvatures. It follows, then, that
measurements of growth rate play an important part in the interpretation. They may be made either on both sides, during the actual curvature, or while the organ is growing straight, under all-round illumination of the same intensity and duration as that causing curvature.
It stands to reason that the only part of an organ which can curve
in response to light is the part which is elongating. Curvatures are
therefore limited to the elongating portions of the seedlings, roots, stems,
fungal hyphae, or other objects used for study. However, in some cases
the limitation is more severe than this, and at least in many seedlings
it is clear that only part of the growing zone is able to respond to light.
There is also the possibility, which cannot be excluded, that the incidence of light may have brought about elongation in a region which
had not been elongating in the dark. For these reasons growth measurements have to be made in detail, and by zones, in order to delimit the
response as much as possible.
249
The relative insensitivity of many animals to the ultraviolet is mainly
due to the powerful absorption of the ultraviolet by the eye lenses,
which act as cut-off filters in this region (15). The data as a whole
suggest the action of several red to purplish light-absorbing systems,
and it is evident that what is being measured is essentially the absorption spectra of a group of "visual purples" (see Chapter 7 by Wald, in
this volume).
III. Phototropism in Plants
So much of the work on phototropism has been in fields remote from
biochemistry that it hardly seems appropriate to present a complete
historical review here. Instead, this presentation will be limited to those
aspects of phototropism which have been treated biochemically or biophysically, or are at least capable of such treatment. An extensive review of early work on phototropism, along with some original material, was presented by du Buy and Nuernbergk in 1932-1935 (16).
Briefer surveys have been given by van Overbeek (17), du Buy (18),
Went and Thimann (19), and Oppenoorth (-20), the last-named author
adding much additional data on the part played by auxin. More recently the subject has been reviewed by Brauner (21), Went (22), and
Galston (23).
A phototropic curvature results from a difference in growth on the
two sides of an elongating organ. If the curvature is positive, i.e.,
towards the light, it could be due to: (a) deceleration of growth on
the lighted side (or a greater deceleration there than on the shaded
side); (b) acceleration of growth on the shaded side (or a greater
acceleration there than on the lighted side); or (c) both. Corresponding
possibilities are offered for negative curvatures. It follows, then, that
measurements of growth rate play an important part in the interpretation. They may be made either on both sides, during the actual curvature, or while the organ is growing straight, under all-round illumination of the same intensity and duration as that causing curvature.
It stands to reason that the only part of an organ which can curve
in response to light is the part which is elongating. Curvatures are
therefore limited to the elongating portions of the seedlings, roots, stems,
fungal hyphae, or other objects used for study. However, in some cases
the limitation is more severe than this, and at least in many seedlings
it is clear that only part of the growing zone is able to respond to light.
There is also the possibility, which cannot be excluded, that the incidence of light may have brought about elongation in a region which
had not been elongating in the dark. For these reasons growth measurements have to be made in detail, and by zones, in order to delimit the
response as much as possible.