386 PLANT PHYSIOLOGY 



do not result in the formation of flowers until the unfavorable 

 conditions of the late summer check the intake of materials from 

 the soil. This lowers the intake of nitrogen while the carbohy- 

 drates increase proportionally, with the result that reproductive 

 organs are formed. This work has many practical results. A vig- 

 orously vegetative, unfruitful plant may be made fruitful by 

 decreasing the nitrogen supply; but it should be noted that if 

 nitrogen is added to a plant lacking it, the result will depend upon 

 the carbohydrates present. If the carbohydrate content is low, 

 addition of the nitrogen will only increase the vegetative growth 

 and have little effect upon fruiting; while if there is a high carbo- 

 hydrate content, fruiting will be fostered. This explains apparent 

 anomalies such as that noted by Hooker in apples and other 

 fruit trees, where the initiation of fruit buds is associated with 

 high carbohydrate content, but the actual fruit development is 

 associated with an increase of nitrogen in the fruit spurs. The 

 C/N ratio also has been found important in Cryptogams (molds), 

 where spore formation is favored by carbohydrates and vegetative 

 growth of the mycelium by nitrogen compounds. 



The nitrogen-carbohydrate balance may not only affect the 

 formation of vegetative structures but may even determine what 

 type of organ (root or shoot) is produced. Nitrogen, as shown 

 by Reid (1926) and others tends to inhibit the formation of roots, 

 while carbohydrates favor their development. Thus Starring found 

 that in tomato cuttings only 0.05 cm. of growth was obtained 

 with a low carbohydrate and high nitrogen content; while with 

 high carbohydrate and low nitrogen content, growth per cutting 

 was 19.7 cm. Also in willow cuttings, there is evidence that at 

 least a part of the polarity observed (Chaps. XVII and XXVIII) 

 is due to differences in the C/N ratio at the root and shoot ends 

 of the cuttings. Roots appear where the ratio is high (least N and 

 most C); shoots where it is low (Hicks, 1928). 



Photoperiodism. — The length of the illumination period and in- 

 tensity of the light also play an important role in the determina- 

 tion of vegetative and reproductive organs as shown by Garner 

 and Allard (1920-1925), who grew several species and varieties 

 of plants at various intensities of light and for various periods 

 of exposure to light. In general, the vegetative growth of the 

 plants varied with the length of the daily exposure to light (or 

 photoperiod) , as might be anticipated. The plants exposed to a 



