PHOTOTROPISM The fact that plants growing near a window will bend their shoots towards the light is familiar to everyone. This reaction is known as phototropism, which means a turning towards light; the term has replaced heliotropism, which means a turning towards the sun, since the reaction is shown in relation to almost any light. As in the case of geotropism there are two types of response, positive and negative, where the turning is towards or away from the source of light respectively. The first is found strongly marked in most shoots of the higher plants and is also found in a few roots. The positive phototropism of most roots is to be expected, since without some such response the light needs of the plant could hardly be satisfied. It is also to be noted that with the ordinary diffuse light of the sun the phototropic reaction is much more powerful than the geotropic. A seedling developing from a seed underneath a stone in a position to which light reaches, will not, under the influence of gravity, push its shoot fruitlessly against the stone above, but will show a phototropic curvature and so reach the light. Negative phototropism is found in some underground roots but markedly in the aerial roots found in many orchids. There are also plant organs which tend to set themselves at right angles to the direction of the incident light ; these show transverse phototropism. The phototropic reaction is usually a growth reaction like the geotropic one. The side which becomes convex grows faster than the normal, and the concave side less fast. Also, as in the case of geotropism, unilateral light may cause a curvature in a mature organ by starting growth in length afresh. All light curvatures are, however, not due to growth changes but in the case of some leaves to the turgor changes of special cells.
Phototropism and the Product Law.—As with geotropism we find that the product law holds ; the effect depends on the amount of light received, i.e., on the product of the intensity of light multiplied by the time of illumination. The time required with different intensities of light to start a reaction, i.e., the presentation time, is given below for the coleoptile (the seedling sheathing leaf) of the oat :— It is seen that the product varies about the number 20, which means that the oat coleoptile will respond to 20 metre-candle seconds. The principle of summation is also found with photo tropism ; stimuli too weak to cause a reaction will act if repeated.
The intensity of the light required to produce an effect for a given time is found to vary with the colour, in other words, with the wave-length. It is found that visible light of all wave lengths will act, but the violet light is the most effective.
Conduction of Stimulus.—Charles Darwin showed as long ago as 1881 that the light stimulus might be perceived at one place and the curvature result at quite another place, showing that the stimulus was conducted. This is well seen in certain grass seedlings (such as those of Setaria), where the apical cotyledon soon stops growing, while the hypocotyl below continues to elongate for some time. Illumination of one side of the cotyledon alone will bring about a marked curvature of the hypocotyl below. In the same way unilateral illumination of the tip of the coleoptile of the cereal seedling, such as oat, will cause a curvation of the lower part. In relation to the conduction of the phototropic stimulus in this plant a very surprising result was obtained by Boysen-Jensen in 1913. He found that if the tip of the coleoptile be cut off and stuck in again with a thin layer of gelatine and the tip be then exposed to one-sided light a curvature will ensue in the darkened lower region. Later this was shown to be true for other grasses and also for the conduction of a stimulus due to injury. Snow in 1923 was able to show that the same held for the geotropic re sponse of bean roots. If the tip is cut off and is fixed on again with gelatine the roots curve when placed horizontal. This sug gests something which can pass a layer of gelatine, i.e., very definitely a soluble growth-promoting substance which differs from the illuminated tip and causes the curvature. It has also been found that the slowing of the growth of a vertical coleoptile caused by decapitation can be largely removed by replacing the coleoptile in position. Such observations as these have led to a large amount of work, particularly in Holland and Germany. The most striking of the results obtained has been that of F. Went (1927), who has shown that by placing cut off illuminated coleop tile tips on small blocks of agar (a gelatine-like material obtained from seaweed) the growth-promoting substance diffuses into the agar. Blocks of this agar when placed on the cut surface of the coleoptile are able to cause curvatures.