320 M or pho genetic Factors 



were produced but the total leaf area was smaller and there were fewer 



roots. 



Deats ( 1925 ) , studying tomato and pepper, found that the amounts of 

 both phloem and xylem varied directly with the length of day. 



Garner and Allard ( 1923 ) made the significant observation that as the 

 photoperiod becomes less favorable for vegetative growth the structure 

 of the plant becomes somewhat xeromorphic— the stem tends to become 

 more branched, underground parts to enlarge, pubescence to increase, 

 abscission layers to cause leaf fall, and flowers to appear. 



In a study of heterosis in beans, Malinowski (1934) crossed two races 

 and grew them and their hybrids under long and short days. In long days 

 the Fx plants were larger in every way than the parents. In short days 

 they were about the size of the parents and flowered 6 weeks earlier than 

 under long days. This acceleration of flowering seems to have cut down 

 their vegetative growth and reduced heterotic vigor. 



Mac Vicar and Struckmeyer (1946) grew soybeans with a deficiency 

 of boron in different photoperiods. The deficiency symptoms were much 

 more severe under long-day treatment than under short. The boron 

 content of all the plants was much the same, and these authors believe 

 that the effect of day-length was to alter the boron requirement of the 

 plants. 



The relative size of the leafy shoot to roots or tubers is markedly in- 

 fluenced by day -length. Radish, for example, grown under short days, 

 as in a greenhouse in the winter, forms a very large root and a small 

 shoot, but in the longer days of spring the root is relatively much 

 smaller. Other plants with storage roots behave in the same way, as do 

 potatoes in the ratio of tops to tubers ( Pohjakallio, 1953). Zimmerman 

 and Hitchcock (1929) observed that in dahlias short days produce heavy, 

 fleshy root systems but long days, fibrous ones. These workers also found 

 ( 1936 ) that growing Jerusalem artichokes under short days stimulated 

 tuber production but that the same result was obtained by subjecting 

 only the tip of the stem to short days by capping it part of the time with 

 black cloth. This indicates that the control of tuberization by day-length 

 is centered in the growing tip and its young leaves. In general, the 

 growth of underground storage regions is stimulated by day-lengths 

 different from those favorable for the vegetative growth of the shoot. 

 Where the photoperiod is such that the shoot in its growth is unable to 

 use carbohydrate beyond a certain amount, this accumulates in storage 

 regions. Jenkins (1954) reports that long days and relatively high tem- 

 peratures are necessary for bulb formation in shallots. In Poa, long days 

 favor the growth of bulbils and short days of ears ( Schwarzenbach, 

 1956). 



In potatoes (Chapman, 1958), short photoperiods induce tuber for- 



