Nature of Phototropic Reaction.—The earlier view of the positive response of stems to unilateral light was that the side towards the light being more highly illuminated had its growth rate reduced more than did the other side, with the result that the stem bent towards the source of light. Later, however, when the negative response of other plant organs was observed, and the response of transparent structures like root hairs and f ungal hyphae, the view was widely held that the plant responded to the direction of the incident light. A. H. Blaauw was the first to re turn to the simpler theory; he showed that simple unicellular structures like the sporangiophone of Pkycomyces and the com plex hypocotyl of the sunflower exhibited a complicated but definite "light-growth reaction" when equally illuminated all round, the rate of growth being increased in Phycomyces and reduced in the sunflower. Light apart from direction has thus an accelerating effect in the one case and a retarding effect in the other. The turning of the hypocotyl of the sunflower towards the light is easily explicable as the side towards the light would be more illuminated and so grow more slowly. In Phycomyces, however, a negative phototropism might be expected, since the "light-growth reaction" is one of retardation, but in fact the curvature is towards the light. Blaauw, however, pointed out that the glass-like cylin drical structure of the sporangiophore acts like a lens and focuses the light on the further side, which is thus more intensely illumi nated than the nearer. As a result of his work Blaauw put for ward the view that in general the phototropic reaction is simply a growth response to differences of light intensity on the two sides of the organ.
The chemotactic movements of antherozoids and bacteria and the chemotropic movements of roots cannot here be dealt with, but a few words must be said on the so-called nastic movements of plant organs. These are movements which may be called out by a change in external conditions, but, as already stated, the nature of the movement is not determined by the stimulus from outside. The perianth leaves of the tulip exhibit a thermonastic movement, for when the temperature is raised these leaves open as a result of the greater growth of the upper sides. Many flowers show nyctinastic movements, opening and closing in response to changes of light intensity. Similar movements, the so-called sleep movements, are also known in leaves, especially in the leaves of Leguminosae.
Photoperiodism.—A very interesting reaction to light, which is in no sense a phototropic response, has been discovered of late years. It is a familiar fact that at least in temperate climates many plants flower only at certain periods of the year; there are so-called spring flowers, autumn flowers and plants which flower in the summer. This marked seasonal effect must be due to some
varying external condition or set of conditions. It has often been supposed that temperature plays a large part in the development of flowers, and a certain degree of warmth is essential for growth, yet altering the temperature alone will not markedly alter a plant's flowering period. Asters and chrysanthemums cannot be made to flower in summer by lowering the temperature, nor irises in winter by putting them in a greenhouse.
Apart from temperature there is one regular cyclical change associated with the march of the seasons and that is the change in length of the day and night, the day in the latitude of London varying from 161 hours in June to a little under eight hours in December. It was shown by Garner and Allard that in the case of many plants it is the length of the day which is the decisive factor in fixing the season of flowering. The discovery was made in the United States, where the behaviour of a valuable variety of to bacco, known as Maryland Mammoth, was being studied. It had been found impossible to obtain seeds from this variety since it went on growing steadily through the season, sometimes reaching a height of 12 ft., being eventually cut down by frost before it had formed flowers. One autumn a specimen was transplanted to a greenhouse, where it promptly flowered and set seed. This was at first thought to be an effect of temperature, but further investi gation showed that it was impossible to cause the plants to flower in summer. It was soon found that the dominating factor was length of day; if growth conditions were favourable it would flower in the short days of late autumn or winter but not in the long days of summer. By artificially shortening the day to 12 hours, by placing the Maryland Mammoth plants in the dark during some of the daylight hours, flowering could be brought about at any time of the year. Following up this discovery Garner and Allard investigated at Washington the behaviour of a large number of plants. These were grown in pots which were borne upon light trucks running on lines so that the pots could be run in and out of sheds and thus be illuminated for various frac tions of the daylight period. Many plants, such as the tobacco variety mentioned above, soya beans, asters, chrysanthemums, poinsettia, were found to be "short day" plants, and would only flower when the period of daylight was reduced to 12 hours or less. If the time of illumination is suitably adjusted the plants will flower and fruit when they are quite diminutive, while other individuals exposed to full daylight grow to a large size without any trace of flower production. Other plants such as grasses have been shown to be "long day" plants, which require a long daily period of illumination for the initiation of flowering.