CHAPTER X 



PHOTOTROPISM 



Principal Facts 



Everyone has noticed that an indoor plant placed near a 

 window grows towards the light and that it has to be turned 

 every day to keep the growth symmetrical. A simple experi- 

 ment can be made with oats sown in a box receiving the day- 

 hght laterally. The seedhngs shoot easily and instead of 

 growing vertically are all inchned towards the direction from 

 which the Hght comes. Light therefore has a considerable 

 influence on the growth of plant cells. 



These facts may be compared with the great difference in 

 appearance between potato shoots kept in the light and those 

 kept in the dark. In the hght, they remain short, squat and 

 coloured; in the dark, they stretch out into thin stems and 

 have the characteristics known as etiolation, although there 

 is no lack of nutritive substance because the tuber is full of it. 

 But among the many phenomena distinguishable in etiolation 

 we are concerned here only with the rapid growth in dark- 

 ness. 



When a plant is illuminated on one side only, this side 

 grows less quickly than the other which is in the shade, hence 

 a curvature is produced which directs the tip of the stem 

 towards the light. This influence of light on the incUnation of 

 the tip of the stem is called phototropism. 



Owing to phototropism, plant stems are directed from 

 regions with less illumination towards those with more. Thus, 

 parts of the same plant, hke the branches of a tree, or different 

 plants in a group, like the blades in a field of unripe corn, do 

 not bunch themselves together in the same place, but, on the 



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