Temperature 339 



between temperature and the photoperiod (Owen, Carsner, and Stout, 

 1940), a technique which has been termed photothermal induction. 



The problem of dormancy, of why plants or parts of them fail to grow 

 until particular conditions are satisfied, has implications for morpho- 

 genesis, particularly in relation to buds, since there are usually very many 

 buds on a plant that do not develop. Whatever determines the particular 

 ones that are to grow has an important influence on the form of the plant. 

 Factors that inhibit or stimulate growth of these meristematic regions 

 have been studied chiefly in connection with growth substances (p. 386), 

 but others are involved. Among them temperature has an important place. 

 It is well known that low temperature is one of the most effective means 

 for breaking the dormancy of seeds, buds, and other plant parts. The 



30 -""Vday 



20- 



10 



day temperature 16.5* 

 night temperature as 

 indicated on abscissa 



temperature, constant 

 day and night 



3k>- 2S° 20° 15° 10° S'C 



Fig. 15-2. Thermoperiodicity in tomato. Stem growth (in millimeters per day) in 

 plants kept constantly at the indicated temperature ( lower curve ) and for 8 hours dur- 

 ing the day at 26.5 °C but at night at the temperature indicated (upper curve). (From 

 Went. ) 



influence of temperature on bud growth has been widely studied by 

 horticulturists because of its practical importance (see Chandler et al., 

 1937). 



Vernalization. A more significant effect of temperature for morpho- 

 genetic problems is evident in the processes of vernalization, by which 

 flowering is accelerated through the application of low temperature at a 

 particular stage of development. Horticulturists have known for many 

 years that the chilling of seeds or seedlings will in many cases force 

 plants into bloom earlier than would otherwise occur. Scientific study 

 of this effect, however, began in comparatively recent years and at first 

 was explored chiefly by Russian plant physiologists, especially Lysenko. 

 A conspicuous example of the effect of vernalization is the speeding up 



