Control by Need for Chilling 169 



The seeds of some plants must be chilled under moist conditions 

 before they will germinate properly. In other species, such as winter 

 wheat, satisfactory development takes place only if a period of chilling 

 occurs during or after germination. It has been discovered that the 

 seeds of certain plants must undergo two successive cold intervals 

 before the seedling will grow (Barton, 1944). This requirement 

 means that under natural conditions development of the plant can go 

 forward only after the second winter. 



In nature all species requiring low temperatures are limited in their 

 altitudinal distribution down the sides of mountains and in their 

 latitudinal extension toward the equator. This cold requirement also 

 imposes serious restriction on the successful cultivation of temperate 

 varieties of fruit trees in warm climates. Although little is known of 

 the physiological mechanisms upon which the need for chilling de- 

 pends, the empirical knowledge is put to practical use through 

 vemalization, that is, the artificial exposure of seeds to cold. Such 

 procedure allows certain crop plants to be grown farther north and 

 also farther south than would otherwise be possible. 



Another need for low temperatures results from the varying effect 

 of this factor on anabolic and catabolic processes. We have seen that 

 sufficient heat must be available to plants and animals so that the 

 growth and reproductive processes may make up for the destruction 

 of tissue materials and the death of individuals. As temperatures rise, 

 most vital reactions are accelerated, but sometimes at very different 

 rates. If catabolic processes are speeded up disproportionately, the 

 organism will suffer rather than gain from high temperatures. Above 

 certain temperatures the respiration rate of most plants becomes 

 higher than the rate of photosynthesis. Food manufacture is conse- 

 quently curtailed, and growth, reproduction, and the accumulation 

 of food reserves may be inhibited. The range of plants toward low 

 latitudes and low altitudes and their seasonal activity are limited to 

 areas and periods when temperatures remain low enough to permit 

 a favorable photosynthesis-respiration relationship. 



Although the need for low temperatures during part of the life cycle 

 has been extensively studied in relatively few animals, there is little 

 question that failure to meet this thermal requirement limits the dis- 

 tribution of some species. Desert mammals and insects that are 

 forced to aestivate by high temperatures would not be able to con- 

 tinue active life if some portion of the year did not have a cooler cli- 

 mate. The gemmules of sponges hatch at a much lower temperature 

 and in a much shorter time after a period of freezing. The eggs of 



