GROWTH 305 



light on growth is so great, that it creates a definite daily period- 

 icity. During the night plants grow more rapidly than during the 

 day, in spite of the lower nocturnal temperature. Only at a very 

 sharp drop of temperature the accelerating influence of absence of 

 light fails to overcome the retarding influence of lowered tempera- 

 ture, a condition leading to stunted plants as exemplified by the 

 vegetation of the tundra. Low temperatures at night have an 

 influence even on the growth of alpine plants, adding their effect 

 to the influence of bright light in producing increased dwarfing. 



The checking effect of light on growth manifests itself markedly 

 in many microorganisms which even in diffused light develop 

 poorly and die in direct sunlight. Upon this fact is based the 

 sterilizing value of light, so well known in hygiene and medicine. 

 Gloomy, damp, and badly aired rooms are the breeding places of 

 disease germs. Light is also an excellent disinfectant of the 

 human skin, which explains, in part at least, the beneficial value of 

 sun baths. In this respect, of the greatest importance are the ultra- 

 violet rays. Hence in recent years light from quartz mercury- 

 vapor lamps, which is rich in ultra-violet rays, has been used as a 

 substitute for sunlight. 



In prolonged experiments with green plants, the growth- 

 inhibiting influence of light may be masked by its indirect effect. 

 Being a source of the necessary energy for the production of organic 

 matter, light promotes nutrition and consequently better growth 

 of plants. Therefore, in well-lighted places, plants generally de- 

 velop better than in shaded ones. Thus, for instance, plants will 

 grow better in a room when kept near a window than when farther 

 back from it. The light requirements of the various plants vary 

 greatly, as has been seen in Art. 12. A light intensity which is 

 insufficient for some plants may be excessive for others. The 

 anatomical and morphological differences between light and shade 

 plants, considered in the same paragraph, are due more to the 

 indirect than the direct effect of light. The considerable increase 

 of transpiration in light is of particular significance in this respect. 



91. Influence of Moisture on Growth of Plants. — The amount 

 of moisture in the environment has a profound effect on the 

 growth of plants. The greatest increase in volume of the cell 

 occurs, as has been seen, during the stage of elongation. It is 

 largely due to the water which enters the cell. This stage there- 

 fore can be normally attained only when water has an uninter- 



