250
KENNETH V. THIMANN AND GEORGE M. CURRY
Ever since the experiments of Charles and Francis Darwin in 1880,
extensive use has been made of rapidly elongating seedlings for phototropic studies. Such seedlings, especially of the grasses, are, as the
Darwins showed, exceedingly sensitive to light. Obviously seedlings
have the advantages that they can be had in large numbers and can be
handled in a small space. But their main advantage is that they can be
grown in the dark, the cotyledons supplying all nutrients, and they can
then be illuminated under controlled conditions. The majority of the
critical experiments have been done on such material. The important
reservation has to be made, however, that if growth is to be measured
before or after the light treatment, some kind of weak illumination must
be supplied in order to make the measurements. There are, it is true,
auxanometers such as those of de Bouter (24) and Idle (25) which operate in complete darkness, but many of the published measurements
involve some light, and this in turn may have influenced the phototropic reaction. For instance, red light treatment one hour before phototropic exposure reduces the sensitivity of Avena coleoptiles to blue
light by a factor of two or three times (26). In a few cases, isolated
parts of plants have been made use of, and here too some light is in
general necessary in order to carry out the surgical operations.
Other external influences may also modify the response to light.
Nutrition, temperature, water supply, and the humidity of the air are
obvious external factors which must in general be standardized for the
best results. Among internal factors, two are of major importance: (a)
pigmentation, which will not only influence light absorption directly,
but which itself may be modified by preillumination and perhaps by
other treatments; (b) the age of the plant and the accompanying variations in growth rate and in the distribution of growth zones.
In summary, experimental work on the phototropism of seedlings
has six aspects: (1) the observation and measurement of the reaction
itself; (2) the influence exerted on the reaction by external variables,
including illumination before and afterwards; (3) the effects of internal
factors; (4) the measurement and localization of growth responses associated with the reaction; (5) determination of the influence of the intensity, duration, and wavelength of the light; (6) the proposal of interpretations and hypotheses, and special experiments designed to test
these proposals.
All the above applies equally well to roots as to shoots, but the
phototropism of roots is generally much less marked than that of shoots.
As a rule it is either absent or negative in sign, which has obvious
ecological advantages for the growing plant. Mainly because of their
relatively weak response, phototropism of roots has been little studied.
KENNETH V. THIMANN AND GEORGE M. CURRY
Ever since the experiments of Charles and Francis Darwin in 1880,
extensive use has been made of rapidly elongating seedlings for phototropic studies. Such seedlings, especially of the grasses, are, as the
Darwins showed, exceedingly sensitive to light. Obviously seedlings
have the advantages that they can be had in large numbers and can be
handled in a small space. But their main advantage is that they can be
grown in the dark, the cotyledons supplying all nutrients, and they can
then be illuminated under controlled conditions. The majority of the
critical experiments have been done on such material. The important
reservation has to be made, however, that if growth is to be measured
before or after the light treatment, some kind of weak illumination must
be supplied in order to make the measurements. There are, it is true,
auxanometers such as those of de Bouter (24) and Idle (25) which operate in complete darkness, but many of the published measurements
involve some light, and this in turn may have influenced the phototropic reaction. For instance, red light treatment one hour before phototropic exposure reduces the sensitivity of Avena coleoptiles to blue
light by a factor of two or three times (26). In a few cases, isolated
parts of plants have been made use of, and here too some light is in
general necessary in order to carry out the surgical operations.
Other external influences may also modify the response to light.
Nutrition, temperature, water supply, and the humidity of the air are
obvious external factors which must in general be standardized for the
best results. Among internal factors, two are of major importance: (a)
pigmentation, which will not only influence light absorption directly,
but which itself may be modified by preillumination and perhaps by
other treatments; (b) the age of the plant and the accompanying variations in growth rate and in the distribution of growth zones.
In summary, experimental work on the phototropism of seedlings
has six aspects: (1) the observation and measurement of the reaction
itself; (2) the influence exerted on the reaction by external variables,
including illumination before and afterwards; (3) the effects of internal
factors; (4) the measurement and localization of growth responses associated with the reaction; (5) determination of the influence of the intensity, duration, and wavelength of the light; (6) the proposal of interpretations and hypotheses, and special experiments designed to test
these proposals.
All the above applies equally well to roots as to shoots, but the
phototropism of roots is generally much less marked than that of shoots.
As a rule it is either absent or negative in sign, which has obvious
ecological advantages for the growing plant. Mainly because of their
relatively weak response, phototropism of roots has been little studied.