Jochalge. Der plattenförmige Plastid führt phototropische Bewegungen durch: Bei Schwachlicht ist er flächenständig, bei Starklicht kantenständig. Demonstration von Bewegungsabläufen bei Licht unterschiedlicher Wellenlänge: Drehung, Phytochromeffekt mit Induktion und anschließender Aufhebung durch Hell- und Dunkelrot; Wirkungsdichroismus der Schwingungsrichtung des elektrischen Lichts.

Movement of the flat chloroplast in the Mougeotia cell with its orientation towards light by turning around the axis of the cylindrical cell. The chloroplast orients in strong white and blue light as to show its profile to the light source, but in red or low intensity white light, it exposes its face to the light. Both types of movement are demonstrated in the film, showing at the same time that the chloroplast not always responds as a unit to the light, but that the response is restricted to the illuminated part of the cell. The red light effect acts as an "induction", i. e. response is initiated and oriented by a light pulse which continues to act afterwards in the dark. An example is given to show that this induction by red can be reversed by far-red following red; hence phytochrome is the photoreceptor pigment responsible for this type of chloroplast orientation in Mougeotia. Finally, it is shown that in polarized red light the response strongly depends on the orientation of the electric vector of the light in relation to the cell axis.