120 



PHYSICAL FACTORS: LIGHT 



vegetative growth. Since these short wave lengths are more readily 

 absorbed by atmosphere, and especially by clouds, than the longer 

 ones, the quality of the light varies greatly between high and low 

 altitudes and between clear and cloudy weather. 



73. The Effect of Length of Day.— At the equator the days are 

 always twelve hours long and during the summer of either hemis- 

 phere the length of the days increase toward the pole until at the 

 pole light is continuous throughout the summer. The long days of 

 the north, during the summer of the northern hemisphere, enable 

 plants to develop rapidly and mature quickly. It is largely because 

 of the longer days that oranges are ripe and ready for the market 

 several weeks earlier in the northern part of the Central Valley of 

 California than they are 400 miles farther south. It is also primarily 



Fig. 56. — Clusters of mushrooms {Collybia velntipes) that grew on the lower side 

 of a horizontal tree trunk. The curved stems are due to a response to gravity. 



due to the long periods of sunshine during the growing season that 

 large crops of hay, wheat, potatoes, and other vegetables can be 

 grown in some parts of Alaska. 



Experiments have shown that some seed plants normally produce 

 flowers only when the days are relatively long and so may be spoken 

 of as long-day plants. Others come into bloom only when the days 

 are relatively short and are called short-day plants. Long-day 

 plants, such as evening primrose, red clover and radish, can be made 

 to bloom earlier than they normally would by lengthening the day 

 by means of artificial light. Short-day plants, on the other hand, 

 such as tobacco, dahlia and ragweed respond in just the opposite 

 way from the long-day plants and, therefore, in order to bring them 

 into bloom earlier it is necessary to shorten the period of illumination 

 by placing the plants in a dark place during a part of each day. 



