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KENNETH V. THIMANN AND GEORGE M. CURRY
sensitive organ ("dermatoptic sense"). The green flagellates and the
zoospores of multicellular algae, which are classical objects for the study
of movements initiated and directed by light, probably perceive light
wholly or mainly by means of a photosensitive spot at the base of the
flagella, or perhaps by an object called the stigma, a reddish spot near
the anterior end and suggestively close to the base of the flagellum
which causes the movement [see the excellent discussion of the evidence
for this by Halldal (3)]. Light sensitivity is even found in the purple
bacteria, but whether perception in them is limited to a particular spot
or area is not known and indeed there is good evidence that the lightdirected movement here is a function of the rate of photosynthesis (see
Section VI, A).
In all these organisms the response to light is a free movement of
the whole body directed towards or away from the light, and this is defined as phototaxis. In fungi and higher plants, as also in the colonial
hydroids, the body is anchored at one end and the response to light is
shown by a curvature. It is this which is phototropism properly speaking, and because its analysis has gone further in recent years than that
of phototaxis the bulk of this chapter will be devoted to it. Zoologists
have, it is true, used the term phototropism for free movements of animals, while botanists have been more precise in preserving the distinction, but the distinction is a valuable and even essential one to make,
since (regardless of the means of detecting the light) the response to
it is effected in an entirely different way in the two cases. For in
Phototaxis the light influences the organs of movement, muscles, flagella
and the like, while in Phototropism it influences the growth of the
organism.
Experiments and observations on light-directed movements have a
long history. That plants grow towards the light must have been known
to horticulturists centuries ago, and was discussed by Ray in 1693 and
by Hales in 1727. In modern science it was Reaumur in 1740 who first
recorded that insects are attracted by light, while Trembley in 1744
made comparable observations in hydroids.
It was indeed Trembley, a noted contemporary of Reaumur, who
carried out what was almost certainly the first experimental study of
phototaxis (4). After putting a number of hydras into a glass of water
on the table near the window, "Je remarquai un jour," he says, "qu'un
grand nombre de Polypes etoit rassemble sur le cöte du verre tourne
vers le jour. Je fus d'abord curieux de savoir si ce fait n'etoit qu'accidentel, ou bien s'il resultoit d'un penchant marque que les Polypes
eussent pour Tendroit du verre le plus eclaire. Pour m'en assurer, je fis
faire un demi tour au verre. Le grand nombre de Polypes se trouva alors
KENNETH V. THIMANN AND GEORGE M. CURRY
sensitive organ ("dermatoptic sense"). The green flagellates and the
zoospores of multicellular algae, which are classical objects for the study
of movements initiated and directed by light, probably perceive light
wholly or mainly by means of a photosensitive spot at the base of the
flagella, or perhaps by an object called the stigma, a reddish spot near
the anterior end and suggestively close to the base of the flagellum
which causes the movement [see the excellent discussion of the evidence
for this by Halldal (3)]. Light sensitivity is even found in the purple
bacteria, but whether perception in them is limited to a particular spot
or area is not known and indeed there is good evidence that the lightdirected movement here is a function of the rate of photosynthesis (see
Section VI, A).
In all these organisms the response to light is a free movement of
the whole body directed towards or away from the light, and this is defined as phototaxis. In fungi and higher plants, as also in the colonial
hydroids, the body is anchored at one end and the response to light is
shown by a curvature. It is this which is phototropism properly speaking, and because its analysis has gone further in recent years than that
of phototaxis the bulk of this chapter will be devoted to it. Zoologists
have, it is true, used the term phototropism for free movements of animals, while botanists have been more precise in preserving the distinction, but the distinction is a valuable and even essential one to make,
since (regardless of the means of detecting the light) the response to
it is effected in an entirely different way in the two cases. For in
Phototaxis the light influences the organs of movement, muscles, flagella
and the like, while in Phototropism it influences the growth of the
organism.
Experiments and observations on light-directed movements have a
long history. That plants grow towards the light must have been known
to horticulturists centuries ago, and was discussed by Ray in 1693 and
by Hales in 1727. In modern science it was Reaumur in 1740 who first
recorded that insects are attracted by light, while Trembley in 1744
made comparable observations in hydroids.
It was indeed Trembley, a noted contemporary of Reaumur, who
carried out what was almost certainly the first experimental study of
phototaxis (4). After putting a number of hydras into a glass of water
on the table near the window, "Je remarquai un jour," he says, "qu'un
grand nombre de Polypes etoit rassemble sur le cöte du verre tourne
vers le jour. Je fus d'abord curieux de savoir si ce fait n'etoit qu'accidentel, ou bien s'il resultoit d'un penchant marque que les Polypes
eussent pour Tendroit du verre le plus eclaire. Pour m'en assurer, je fis
faire un demi tour au verre. Le grand nombre de Polypes se trouva alors