Control hij Extremes 163 



germination, and at that time killing will result from temperatures 

 that are not nearly as low as those which the plant successfully with- 

 , stood during the winter while it was in a dormant condition. An un- 

 expected temperature relationship is exhibited by certain plants in 

 northwest Europe that can extend their range farther poleward in the 

 interior of the continent than they can along the sea coast. The ex- 

 planation is that the milder winter and earlier rise of temperature 

 near the ocean stimulates the plants here to earlier germination, but 

 s' ice the spring temperatures rise more slowly the seedlings may be 

 killed by late frosts. 



Among those species whose geographical range is limited by tem- 

 perature extremes we find a great variation as to the part of the life 

 cycle that is chiefly concerned. The limitation is brought about by 

 the most susceptible stage; the boundary of distribution is determined 

 by the weakest link in the life cycle in respect to temperature tol- 

 erance. Other life stages may extend temporarily beyond the area 

 of permanent existence. 



We often think of the youngest organisms as the most sensitive to 

 harmful influences in the environment, and this is indeed sometimes 

 true in respect to temperature. The early stages of development 

 often exhibit a very narrow range of temperature tolerance and may 

 thus represent the "heel of Achilles" for the species as far as damage 

 from excessive heating or chilling is concerned. For example, fish 

 eggs characteristically show a greater susceptibility to hann from 

 temperature than adult fish of the same species. The eggs of birds, 

 although a special case because birds are homoiotherms, will tolerate 

 only an extremely narrow fluctuation in temperature and frequently 

 die as the result of unusually cold or hot weather. In some animals, 

 on the other hand, the egg is far more resistant to temperature ex- 

 tremes than any other stage. In other species the most resistant stage 

 is found later in the life cycle and may be represented by a cyst or 

 pupa. The seed of the higher plants similarly represents a tough, 

 resting stage and one that is characteristically able to withstand a 

 range of temperature far greater than that tolerated by the adult plant. 

 Sprouts can often survive a frost that would critically injure the re- 

 productive tissues of the mature plant. 



Very frequently the greatest susceptibility to heat or cold damage 

 lies in the intermediate stages of growth. The period after the egg 

 has hatched or after the seed has germinated is a critical one in some 

 species; it represents the stage at which pioneering individuals at the 

 margins of their range will be killed off by adverse temperatures. 

 The fate of copepod populations carried by currents each year from 



