6. PHOTOTROPISM AND PHOTOTAXIS
251
It has been shown, however, that the negative curvature is associated
with an acceleration of growth by light (27).
Next to seedling shoots, the objects of study most used have been
fungi. The mycelium growing more or less horizontally on nutrient
medium may show some photosensitivity, but it commonly does not;
what is exceedingly light-sensitive is the vertically growing sporangiophore or spore bearer. Those of Pilobolus and Phycomyces have been
the most used. The former forcibly expels the spore mass when mature,
which has led to its use in elegant demonstration experiments (28), but
the latter is easier to culture under sterile conditions and has been more
widely adopted for critical work. These sporangiophores differ, of
course, from the seedlings in being unicellular or coenocytic, while the
seedling is a tissue of many thousands of cells.
IV. Analysis of the Phototropic Reaction
A. THE Two TYPES OF PHOTOTROPISM IN Avena COLEOPTILES
At present, biochemical interest in phototropism is centered on
three main aspects: the role of auxins, the nature of the photoreceptors,
and the biochemical mechanisms suggested by the various kinds of
phototropic reactions. The phototropic behavior of the Avena (oat)
seedling is by far the best described and includes most of the types of
phototropism generally encountered; an outline of its responses is therefore essential before any biochemical interpretation can be offered.
Under natural conditions an Avena seedling will grow toward a
source of white light. If this source is not on the line of the plant's axis,
a phototropic curvature will result. The magnitude of this curvature is
only determined by the original orientation of the plant axis with respect to the light, providing the light operates continuously and there
are no physical obstructions. Thus, Darwin (29), observing the curvatures in a row of seedlings continuously exposed at various distances
from a dim source of white light, noted that the response was not particularly graded with respect to intensity; i.e., curvatures in plants far
from the source were not very different from those close to the source.
Curvatures under continuous exposures begin to appear a half hour or
so after the beginning of the exposure, near the tip of the coleoptile.
They increase steadily until eventually the whole plant is bent toward
the light in a smooth arc.
The early discoveries showed that a very dim source of white light
would suffice for curvatures if the exposures were long. Czapek (SO)
found it necessary to expose seedlings to the light for at least seven
minutes in order to obtain an observable reaction subsequently (7 min-
251
It has been shown, however, that the negative curvature is associated
with an acceleration of growth by light (27).
Next to seedling shoots, the objects of study most used have been
fungi. The mycelium growing more or less horizontally on nutrient
medium may show some photosensitivity, but it commonly does not;
what is exceedingly light-sensitive is the vertically growing sporangiophore or spore bearer. Those of Pilobolus and Phycomyces have been
the most used. The former forcibly expels the spore mass when mature,
which has led to its use in elegant demonstration experiments (28), but
the latter is easier to culture under sterile conditions and has been more
widely adopted for critical work. These sporangiophores differ, of
course, from the seedlings in being unicellular or coenocytic, while the
seedling is a tissue of many thousands of cells.
IV. Analysis of the Phototropic Reaction
A. THE Two TYPES OF PHOTOTROPISM IN Avena COLEOPTILES
At present, biochemical interest in phototropism is centered on
three main aspects: the role of auxins, the nature of the photoreceptors,
and the biochemical mechanisms suggested by the various kinds of
phototropic reactions. The phototropic behavior of the Avena (oat)
seedling is by far the best described and includes most of the types of
phototropism generally encountered; an outline of its responses is therefore essential before any biochemical interpretation can be offered.
Under natural conditions an Avena seedling will grow toward a
source of white light. If this source is not on the line of the plant's axis,
a phototropic curvature will result. The magnitude of this curvature is
only determined by the original orientation of the plant axis with respect to the light, providing the light operates continuously and there
are no physical obstructions. Thus, Darwin (29), observing the curvatures in a row of seedlings continuously exposed at various distances
from a dim source of white light, noted that the response was not particularly graded with respect to intensity; i.e., curvatures in plants far
from the source were not very different from those close to the source.
Curvatures under continuous exposures begin to appear a half hour or
so after the beginning of the exposure, near the tip of the coleoptile.
They increase steadily until eventually the whole plant is bent toward
the light in a smooth arc.
The early discoveries showed that a very dim source of white light
would suffice for curvatures if the exposures were long. Czapek (SO)
found it necessary to expose seedlings to the light for at least seven
minutes in order to obtain an observable reaction subsequently (7 min-