348 TEXTBOOK OF PLANT PHYSIOLOGY 



dry soil, on the other hand, appears to be less favorable to fruiting, 

 than a humid one. Consequently, the conditions that are most 

 favorable to fruiting are those attained on well-irrigated land in an 

 arid climate with a large number of bright days, as, for instance, in 

 California, or Central Asia. Under such conditions fruit trees 

 generally yield very large crops. 



The effects of temperature on fruiting seems to be very com- 

 plex. In general, a higher temperature is required for reproduc- 

 tion than for vegetative growth. It is possible to prevent plants 

 from blossoming for a long period by growing them at a low tem- 

 perature. But a temporary lowering of temperature to near 

 freezing may serve as a stimulus for acceleration of blossoming 

 and fruiting. This is most markedly evident in winter cereals. 

 Under usual cultural conditions they will produce during the first 

 year nothing but vegetative growth, even when sown in the spring, 

 without a tendency to head formation before the beginning of the 

 following summer. If the seeds are germinated at a temperature 

 slightly above zero, however, winter cereals will blossom during 

 the first year and, providing they have been sown sufficiently 

 early, may even produce ripe grain. Lowering of the temperature 

 during germination seems to serve as a stimulus, the effect of which 

 continues even after the causal factor has been removed. Low 

 temperatures produce a similar effect on biennial plants. Instances 

 are known in which a cabbage or a beet, when nipped by frost, will 

 start to fruit during the first year of its life. The mechanism of 

 this effect is unknown. 



Garner and Allard's recent investigations, which have been 

 confirmed by a number of other investigators, show that very 

 great importance in determining the time of blossoming should be 

 attributed to the relationship existing between the duration of day 

 and night. Some plants, as, for instance, many varieties of soy- 

 beans, kidney beans, the asters, and other plants, blossom when 

 the days are short and the nights long. Consequently, under 

 natural conditions these plants will produce flowers only in autumn 

 when the days become shorter. By an artificial shortening of the 

 day, as for instance, by placing the plants into a dark room during 

 part of the day, or by covering them with opaque boxes, they may 

 be made to bloom at the beginning of summer. Such plants have 

 acquired the name of "short-day" plants (Fig. 145). There are, 

 however, many plants, as for instance wheat, oats, and other 



