552 RHYTHMS IN PLANTS AND ANIMALS 



periodic in nature, were mediated by a fairly large number of light- 

 sensitive absorption processes, each with a different action spectrum 

 and presumably depending on different pigments. These morpho- 

 genetic processes, including growth responses of stems and leaves of 

 many plants, seem to be completely separate from the problem of 

 photoperiodism as discussed in this symposium. The second group of 

 light processes depends for its response on the red, far-red pigment 

 system, but also is nonperiodic in nature. Many germination processes 

 fall into this group. And finally the third group of light processes 

 comprises the truly photoperiodic processes, which are periodic in 

 nature, that is to say, which depend on the BUnning cycle, and which 

 seem to be controlled by the red, far-red pigment system. Flower 

 initiation and leaf and stem growth processes belong to this group. 

 The analysis of their behavior can be expressed by Galston's P-I-G 

 abbreviation (p. 137) indicating Photoreceptor-Zntermediate proc- 

 esses-Growth responses. 



There exists another set of periodic responses that can be induced 

 by temperature cycles and that have been termed thermoperiodic 

 phenomena. They can be of a daily or a yearly nature; in the following 

 discussion only the daily thermoperiodic phenomena will be treated. 

 Their analysis can be succinctly expressed by another reference to our 

 sister science as T-I-C, meaning rhermoreceptor-Zntermediate process- 

 Cellular responses. Before going into greater detail in a comparison 

 between T-I-C and P-I-G, some pertinent thermoperiodic phenomena 

 will be discussed. Since most of these thermoperiodic responses relate 

 to vegetative processes, it is perhaps well to point out that photo- 

 periodic responses in plants are by no means restricted to flowering, 

 but that almost any vegetative or biochemical process may be in- 

 volved, such as those discussed by Dr. Galston and Professor BUnning. 

 The action spectra of several of these vegetative photoperiodic proc- 

 esses have been determined; they were found to agree very closely with 

 the action spectra of the photoperiodic flowering responses (e.g., leaf 

 expansion of etiolated pea plants and straightening of the plumular 

 hook in bean seedhngs). 



The flowering of a tomato plant is not affected at all by the length 

 of daily exposure to light, but many of its vegetative responses are 

 strongly influenced. Their daily rhythms can easily be demonstrated 



