SEASONAL PHENOMENA IN THE LIFE OF PLANTS 455 



influence of frost, for our trees shed leaves in autumn even when 

 kept in a room or warm greenhouse. The investigations of 

 Garner and Allard (1920) have shown that a great influence is 

 exerted by the length of the day, Avhich rapidly decreases toward 

 the autumn. By artificially shortening the summer day and 

 prolonging the night by covering the plants wdth dark boxes or 

 moving them into a dark chamber in the morning and evening 

 hours, Garner and Allard observed in many tree species a very 

 early leaf fall. These observations were considerably expanded 

 by Moshkov (1930), who has been able to show that not only 

 abscission of leaves but all the processes of the so-called "autum- 

 nal maturation" of the shoots, such as the cessation of growth, 

 the dormancy of the top bud, the development of a cork layer 

 on the surface of the shoots, and the accumulation of reserve 

 substances in the bark are controlled to a great degree by the 

 relative length of the day and night. If this relationship is 

 changed, the time when trees begin their winter dormancy can 

 be controlled within wide Hmits. 



Dormant organs and tissues of plants as compared with 

 those in a state of active growth are much more resistant to 

 unfavorable external conditions, especially to frost and drought, 

 owing to a decreased water content, an increased amount of 

 reserve substances, and changes occurring in the colloidal state 

 of the protoplasm. Both abscission of leaves and ripening of 

 the wood are reactions of the plant in response to the beginning 

 of drought in tropical climates and to the beginning of winter in 

 moderate latitudes. Likewise, cereals during the period pre- 

 ceding the elongation of the stem, w^hen the organs of reproduc- 

 tion are stifl in an embryonic resting state, are more drought 

 resistant than later on when the stem and spike begin to elongate. 

 Thus, the rest period may be regar.ded as a special adaptation 

 fixed by inheritance for surviving unfavorable seasons of the 

 year. By controlUng the beginning of this period, Moshkov 

 was able to control the frost resistance of tree species. Such 

 southern species as Rohinia pseudoacacia, walnut, and apricot, 

 which usually freeze in Leningrad, overwintered excellently 

 without any protection when grown with a shortened day. This 

 explains the fact why Rohinia pseudoacacia overwinters well in 

 the southern Volga region, where the winter is more severe but 

 the days are considerably shorter than in Leningrad. 



