332 PLANT PHYSIOLOGY 



condition, accompanied by a reorganization of the food reserves 

 in the cells (Lewis and Tuttle, 1923). Hardy varieties of wheat 

 are those with a low moisture content and high osmotic pressures 

 (Martin, 1927); and the same was found to hold for winter rye 

 (Tysdale and Salmon, 1926). These random samples from the 

 recent literature could be duplicated many times. That the effect 

 is related to the precipitation of the proteins is indicated by the 

 work of Newton and Brown (1931), who found that sucrose added 

 to the pressed juice of winter wheat protected it against precipita- 

 tion of the proteins contained up to 8% concentration, a value 

 obtained by hardy varieties in the field; but just how the sugar 

 protects is still somewhat obscure. 



The morphogenic effect of temperature is seen when plants 

 which are grown ordinarily at medium temperatures are trans- 

 ferred to mountain tops where the temperature is much lower, 

 especially at night. To determine just how much of the change 

 in form is due to temperature, the conditions have been simulated 

 in the laboratory by surrounding the plant at night with ice and 

 then exposing it during the day to ordinary temperatures. Plants 

 grown under such conditions have the characters so typical of 

 Alpine forms, — limited growth, tough leaves, short internodes, and 

 early flowering season. 



An interesting morphogenic effect of heat is seen in the case of 

 cabbage, which, when grown continuously at 70° F. does not be- 

 have like a normal biennial but continues to grow vegetatively, 

 reaching a height of over 7 ft. during the second year and having 

 a very different appearance from that of cabbages grown in their 

 natural environment (Miller, 1928). Barley, when grown at 20° C. 

 is large, weak, and sprawling with no flowering shoots (Walster, 

 1920), while at 15° growth is normal. The acetic acid bacterium, 

 B. pasteurianwn, when cultivated at ordinary temperatures (30- 

 35° C.) forms chains of short rods, but when grown at 40° C. the 

 cells elongate to form slender filaments which are 100-150 times 

 as long as the original cells. When returned to the lower tempera- 

 ture, the normal form is resumed. 



Light. — Although light is absolutely essential to growth (since 

 without it photosynthesis stops), it will seem paradoxical that 

 light, especially strong light, really hinders growth. This checking 

 action of light on growth is a complex matter, not entirely re- 

 solved. In the dark, cell division occurs better than in the light 



