TIMING AND THE FLOWERING PROCESS 131 



another often do so in response to the length of day. There are 

 many known examples, some of which are quite spectacular. The 

 pitcher plant midge larva floats in the water of a pitcher plant, a 

 carnivorous plant which digests insects in order to obtain a 

 nitrogen supply. This little mosquito-like creature, however, 

 resists all attempts to be digested and goes through its develop- 

 mental stages in this rather interesting environment. The insect 

 pupates when the day-lengths exceed about 12 hr. The midge 

 responds to light intensities that are at least an order of magnitude 

 dimmer than those to which sensitive, day-length controlled plants 

 will respond. This is near the limits of human vision, so the midges 

 are responding to light when the human observers might agree that 

 it was dark. 



F. The reproductive and migratory cycles of many animals. 

 Both invertebrates and vertebrates, including for example insects, 

 fish, lizards, turtles, birds, and mammals become reproductive only 

 at certain times during the year. Often this is a response to day- 

 length, as was first discovered in 1925 by W. Rowan with slate- 

 colored juncos in Canada. Birds were captured during the summer 

 and kept until fall. Then the day-length was gradually increased 

 with artificial lights to simulate the conditions of spring. The birds 

 responded by undergoing the gonadal swelling typical of spring and 

 exhibiting the symptoms of restlessness which are associated with 

 the desire to migrate. When the juncos were released in Canada 

 in the middle of winter, they flapped their wings and headed north 

 into the blizzard, because they had been physiologically "convinced" 

 by the long days that spring was here and that it was time to fly 

 north. Obviously this was an important step forward in science, 

 but it does seem like a rather unethical trick to play on the birds ! 



G. The color of rodents. Certain animals such as the Arctic hare 

 change color from the brown of summer to the white of winter in 

 response to shortening days. Obviously this is quite closely related 

 to the gonadal development discussed above. 



3. Thermoperiodism 



Greenhouse operators have probably always thought that plants 

 grow best if the natural temperature fluctuations of day and night 

 are simulated. Thus it is common practice to lower the greenhouse 

 temperature at night. Frits Went, while at the California Institute 



