96 PLANT PHYSIOLOGY AND ECOLOGY 



which the plant dies. Naturally, maximum and minimum tem- 

 peratures vary widely for different species, and to a small extent 

 also for different individuals of the same species. Furthermore, 

 a plant can withstand extremes of heat and cold much better in 

 some stages than in others. It is least resistant in the active 

 condition when the tissues are filled with water, and most re- 

 sistant in the resting state typical of spores and seeds. Nearly all 

 flowering plants are killed by exposure to temperatures below 

 C. and above 45 C. Their seeds, however, may resist tem- 

 peratures of 250 C. and 100 C. The resistance of seeds depends 

 in a very large degree upon the absence of water. Seeds when 

 quite dry have been found to survive when exposed for an hour 

 to 110 C, while an exposure to water vapor at 70 C. killed 

 them in one fourth the time. The maxima and minima of natural 

 habitats during the growing period practically always fall between 

 C. and 45 C. In hot springs the maximum for many algsB 

 and bacteria rises to 85 C. or liigher. The bacteria are capable 

 of withstanding much liigher temperatures, and, in the spore 

 condition, are able to resist temperatures of 120-130 C. 



ii8. Freezing. The injury arising from exposures to low 

 temperatures depends primarily upon the amount of water that 

 the plant contains. For example, the watery leaves and stems 

 are usually killed by exposure to C, while the drier seeds and 

 underground parts resist the long-continued action of tempera- 

 tures from 30 to 40 C. Dry seeds, moreover, are capable 

 of germination after exposure to 25 C, but soaked seeds lose 

 this power under such conditions. The reason for this difference 

 of behavior resides chiefly in the protoplasm, though it is impossible 

 to go further than the statement that active protoplasm is more 

 sensitive to cold than is the resting form. The small amount of 

 cell-sap in resting tissues is also much more concentrated than 

 that of living cells, and this of course increases the power of re- 

 sistance. 



Frost, i.e., freezing temperatures, kills plants by withdrawing 

 water from the cell-sap and forming ice crystals, usually upon 

 the outside of the cells. When the freezing is extreme, the cells 

 are ruptured by the formation of crystals within them. It is very 

 probable that the chemical constitution of cell-sap and proto- 

 plasm is changed in a manner harmful to the plant. Such a result 

 seems to be clearly indicated by the behavior of many plants 



