128 THE STUDY OF PLANT COMMUNITIES • Chapter VI 



erably north of the center for corn is therefore not at all surpris- 

 ing. 



Absorption of water is at a minimum when soil is frozen but 

 increases, as do diffusion and capillary movement in the soil, with 

 rising temperature. The optimum is surprisingly high as soil tem- 

 peratures go, and the maximum approaches the boiling point in 

 some instances. Absorption is reduced, more at low temperatures 

 for plants that grow normally in warm soil than for plants that 

 grow, at least part of the year, in cold soil. For example, cotton 

 absorbs only 20 percent as much water at 50° as at 77° F. while 

 collards absorb 75 percent as much at 50° as at 77° E 148 



Photosynthesis operates under a wide range of temperatures 

 under natural conditions. Marine polar algae may live their entire 

 lives at temperatures below 32° F. because the freezing point is 

 depressed by the salts in the water. There is an often-quoted old 

 report that spruce carries on photosynthesis at -22° E, but a re- 

 cent study" using modern methods indicates that, although coni- 

 fers do not lose their ability to carry on photosynthesis during 

 midwinter, the species studied function only above 2 1 ° F. The 

 process also goes on in desert plants at temperatures of 120° E or 

 more. The effective temperature range, however, is usually be- 

 tween 70° and 100° E With increase in temperature the rate in- 

 creases steadily to the optimum and then drops abruptly to the 

 maximum, which is not much in excess of the optimum. The rate 

 of respiration also increases with temperature until at high tem- 

 peratures the process becomes destructive of life. Vant Hoff's 

 Law, which states that the speed of a chemical reaction doubles 

 or more than doubles with each 18° F. rise in temperature, is ap- 

 plicable within limits to reactions in organisms. In photosynthesis 

 it holds reasonably well between about 41° F. and 77° F. Beyond 

 these limits there is much variation. 



Growth, being a product of chemical and physiological proc- 

 esses, follows the same pattern and is favored by relatively high 

 temperatures. At temperatures near or above the maximum, the 

 water balance is apt to be thrown off by excessive transpiration. 

 Reproduction follows the same rule regarding temperature, but 

 it is of interest that flowering and fruiting have higher optima 

 than vegetative processes in the same plant. 



