SummaryThis article is devoted to photochemical reaction systems other than photosynthesis present in plants which enable normal growth and development to take place. In many higher plants (e.g. dicotyledonous seedlings) at least four different photochemical reaction systems are effective:(1) the photochemical system related to photosynthesis,(2) the photochemical system related to phototropism,(3) the phytochrome system,(4) the so‐called ‘high energy reaction of photomorphogenesis’.(1) is beyond the scope of this article which is limited to those photoreactive systems in plants which directly control growth and development.The term ‘phytochrome’ is used today to signify the pigment system of photo‐morphogenesis which has been most thoroughly investigated and which is apparently common to all potentially green plants. From the algae, mosses and ferns to the monocotyledons this pigment system is of fundamental importance. It is composed of a complex chromoprotein present in the cytoplasm, which can be isolated from the cell and brought into watery solution. Phytochrome has two interconvertible forms, phytochrome 660 (P 660) with an absorption maximum in the red at 660 mμand phytochrome 730 (P 730) with an absorption maximum in the far‐red at 730 mμ. The non‐irradiated, dark‐grown plant contains almost entirely P 660 which is stable in the dark. P 660 is converted by exposure to red light into P 730. Conversely P 730 can be reconverted into P 660 by exposure to far‐red light. Even in the dark P 730 slowly changes back into P 660 Temperatures above zero and in the presence of oxygen are necessary. P 730 is the physiologically active form of the phytochrome system. Apparently it is an enzyme. As soon as P 730 is available, reactions occur which cannot proceed without it and which finally lead to the observable photo‐morphogenic responses.In 1957 it was demonstrated that phytochrome cannot be the only photoreactive system in photomorphogenesis and anthocyanin synthesis, and experimental data made it necessary to suppose that a further photoreactive system plays a part. By contrast with phytochrome this other reaction system can be physiologically demonstrated only by irradiating relatively strongly for a long period of time (therefore called high energy reaction). This reaction system is not reversible. Under natural conditions of radiation it seems to be very important; it is apparently as widely distributed as phytochrome. To study the high energy reaction more closely it has been necessary to separate responses due to it from those due to phytochrome. This has been done by the use of rather complicated irradiation programmes, in situations involving either synergism between phytochrome and the high energy reaction (e.g. with mustard seedling) or antagonism (e.g. movements of the plumular hook in lettuce seedlings). The situation becomes more simple when we investigate photoresponses which are not markedly influenced by phytochrome and are largely under the control of the high energy reaction (e.g. hypocotyl lengthening in lettuce seedlings). The known action spectra of this reaction show peaks of the same order of magnitude in the blue and in the far‐red range of the visible spectrum. The mode of action of the high energy reaction is far from firmly established. One hypothesis is, that the activation of an enzyme (e.g. a metal‐flavoprotein) by visible radiation is the basis of the reaction. This enzyme must be of fundamental importance in metabolism because many different photoresponses are controlled through the high energy reaction.In a typical dicotyledonous seedling elongation of the hypocotyl is controlled by the phytochrome and by the high energy reaction. That is, the control of axis growth can be effectively exerted by long‐wavelength visible light (above 600 mμ). A phototropic curvature, however, can be induced only by the shorter wavelengths of visible radiation, i.e. wavelengths below 500 mμ. There is thus no immediate relation between phototropism and control of stem growth by phytochrome and the high energy reaction.The germination of fern spores and the growth of the gametophytes are strongly influenced by light. In recent years it has been possible to demonstrate that several physiologically distinct photoreactive systems control development and morphogenesis during these stages: (1) the photochemical system concerned with photosynthesis, (2) the phytochrome system (germination, partly morphogenesis, partly phototropism), (3) a photoreactive system dependent on blue light mainly controlling morphogenesis and partly phototropism and germination. In the case ofDryopteris filix‐masnormal morphogenesis, i.e. the rapid formation of two‐ or three‐dimensional prothallia, can only occur under short‐wave visible light (blue light). In darkness and under long‐wave visible light (red light) the gametophytes grow as filaments. The control of morphogenesis by a blue‐light‐dependent photoreactive system is connected with the increase of protein synthesis under the influence of the light and with changes in nuclear volume and chloroplast size. These gametophytes are a clear example of the control of nuclear and cellular properties by an external factor, light.Phototropism and polarotropism of the protonernatal filaments can be induced by blue and by red light. The polarotropic response, i.e. control of the direction of growth by the plane of vibration of the electrical vector of linearly polarized light, must be regarded as a variant of phototropism mediated by an orientation of dichroic photoreceptor molecules with respect to the nearby surface. An action spectrum of polarotropism indicates that both phytochrome and the blue‐light‐dependent pho