TIMING AND THE FLOWERING PROCESS 129 



Studies among other things the changes in respiration rates of 

 potato and carrot tissue, a far cry from fiddler crabs. 



E. Growth rates of certain plants. The coleoptile or sheath 

 which covers the first true leaf of a germinating grass seedling has 

 been studied very intensively by plant physiologists because of its 

 growth responses to light and gravity. Among many other things, 

 it seems to grow at rates which vary according to a circadian 

 rhythm. 



F. Root pressure. If tops are removed from plants such as 

 tomato or grape, the cut stump will exude sap under some pressure, 

 providing soil moisture conditions and other factors are right. The 

 amount of sap exuded follows a circadian rhythm which seems to 

 be independent of the environment. 



G. The activity of certain enzymes, the concentrations of certain 

 pigments, etc. Many such factors in plants and animals vary 

 according to circadian rhythms. 



H. Discharge of spores such as those from the fungus Pilobolus. 

 Rhythms are easy to study with such organisms, since they can be 

 grown in total darkness, making it easy to control temperature and 

 humidity. 



I. Bioluminescence. Certain organisms, especially some found 

 in the ocean, give off light according to a circadian rhythm. The 

 marine green algae, Gonyaulax, has been studied in considerable 

 detail. In this case, a population of single-celled organisms is 

 studied rather than one or a few larger organisms. Gonyaulax also 

 exhibits rhythms in cell division and photosynthesis. 



J. Activity of various animals. Rats, hamsters, cockroaches, 

 and a number of other animals have been studied because of their 

 circadian rhythms of activity. Rats, for example, may be placed 

 in automatic feeding and watering cages with an attached treadmill 

 for running. The treadmill can be monitored through a pen 

 recording on a clock-driven drum so that every time the rat 

 exercises, the time and duration of its activity are automatically 

 recorded. A few dozen of these arrangements can be placed in a 

 room, and the records can be examined after a few weeks. Follow- 

 ing a light period, the animals will be active in the darkness (or 

 under low intensity light) about once every 24 hr. Each animal 

 has a very consistent period, which is usually somewhat longer or 



