GROWTH OF PLANTS 135 



that diffuses from the top downward. Others think that Ught 

 retards the synthesis of the growth-promoting substance neces- 

 sary for the elongation of cells. 



The dwarfing effect of light on the aerial organs is particularly 

 marked in high mountains, where, on account of greater trans- 

 parency of the atmosphere, solar light is considerably richer in 

 ultraviolet rays than in valleys. Alpine plants, as the investi- 

 gations of Bonnier have shown, are always of short stature. 

 And when common plants of the valley, e.g., the dandelion or 

 Betonica, are grown in high mountains, they assume the charac- 

 teristics of alpine plants (Fig. 35). The retarding influence of 

 Ught 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. A very 

 sharp drop of temperature may overbalance the accelerating 

 influence of darkness, a condition leading to stunted plants, as 

 exemplified by the vegetation of the tundra. Low temperatures 

 at night have an influence also on the growth of alpine plants, 

 adding their effect to the influence of bright light in producing 

 increased dwarfing. 



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 develop better than in shaded ones. Plants will grow 

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

 back from it, where they may become exhausted. The light 

 requirements of plants vary greatly, and a light intensity that 

 is insufficient for some plants may be excessive for others. 



32. Influence on Growth of the Degree of Saturation of the 

 Plant with Water. — Normal growth processes require saturation 

 of protoplasm with water; for only in a sufficiently mobile 

 protoplasm can there be the free movement of the chromosomes 

 necessary for the process of mitosis and other functions accom- 

 panying the division of cells. The stage of elongation is still 

 more dependent upon the conditions of water supply, for the 

 increase in volume depends upon the absorption of water by the 

 cell. An excellent example of the significance of the degree of 

 saturation with water for growth is presented by seeds. Air- 



