340 PLANT PHYSIOLOGY 



lates it. The retardation seldom lasts more than a day, after 

 which growth is greater than normal. The increase of growth 

 produced by a slight tension may be as much as 20% more than 

 that of the control. The morphogenic effect is generally confined 

 to the production of spiral thickenings on the vessels and a gen- 

 eral increase in the firmness of the organs, which changes un- 

 doubtedly have survival value. 



Gravity. — Gravity influences development chiefly in deter- 

 mining the direction of growth. For this reason a discussion of 

 its influence is reserved for a later chapter. 



Electricity. — The effect of electricity on plant growth has re- 

 ceived much attention of late in an attempt to find some easily 

 applicable method for the electrical stimulation of plants. Most 

 of the work has been done either by placing seeds in an electrolytic 

 bath or by placing the growing plant in an electric field produced 

 by a silent discharge. The results obtained by various workers 

 are often contradictory, but in general it can be said that the 

 electrolytic treatment of seeds accelerates their germination and 

 hastens the development of the seedlings. The treatment of the 

 growing plants has generally resulted in earlier and better crops. 

 Wheat, for example, when so treated produced an increased yield 

 of 39%, and sugar beets were found to produce a 33% greater 

 yield with 1.1% more sugar. Part of this additional growth may 

 be due to the ionization of the air, as previously suggested 

 (Chap. IV). 



QUESTIONS 



1. From the discussion of the morphogenic effects of temperature on barley, 

 would you conclude that barley is a plant of a cold or warm climate? 



2. If Helianthus seeds are heated to a high temperature, the seedlings often 

 develop abnormally. Is this a "morphogenic" effect? 



3. Give two reasons why red light is better than blue light for the growth of 

 plants. 



4. What would be the effect of the half-shade system of propagation upon 

 vitamin content? 



REFERENCES 



Adams, J. — Some further experiments on the relation of light to growth. Am. 



Jour. BoL, 12:398, 1925. 

 Andrews, F. M. — The effect of pressure on growth. Indiana Acad. Set., 



38:265, 1922. 

 Blackman, V. H. — Field experiments in electro-culture. Jour. Ag. Sci., 



14:240, 1924. 



