Morphological and Physiological Ackiptations 145 



"bound" in colloid form with the result that the freezing point is de- 

 pressed below values ordinarily experienced. If you walk through a 

 field of winter rye on a day when the temperature is well below 0°C, 

 you will notice that the leaves and stems are flexible and not brittle as 

 they would be if the tissues were frozen. The completion of these 

 hardening processes takes time, and the actual high or low tempera- 

 tures that an animal or a plant can withstand often depend upon the 

 interval available for acclimatization. Evergreen trees and many 

 other types of plants similarly become "frost hardy" during the winter. 

 Everyone is familiar with the ability of crocuses, snowdrops, and other 

 spring flowers to withstand frost and even to push up through a layer 

 of snow. On one occasion a fully opened crocus blossom was ob- 

 served unharmed after a night during which the air temperature had 

 dropped to — 16°C. Various species of alpine plants exhibit a se- 

 quence in their flowering in or near snowbanks according to their 

 diflering tolerance of low temperatures. Many insects are similarly 

 able to form bound water in their tissues and thus avoid the dangers 

 of freezing. 



Another method by which extremes of temperature are endured 

 involves the removal of water from the tissues. Dried seeds, spores, 

 and cysts avoid freezing because no liquid remains that can freeze. 

 Dry seeds have germinated successfully even after exposure for 3 

 weeks to liquid air (about — 190°C). In another experiment nema- 

 todes and tardigrades are reported to have recovered after chilling for 

 several hours at an even lower temperature. At the other extreme 

 tardigrades in a resistant condition have endured immersion in boiling 

 water for short periods of time, and dried cysts of the ciliate, Colpoda 

 cucullus, have survived after exposure to dry heat at 100°C for 3 days. 

 The even greater thermal resistance of some bacterial cysts is well 

 known. Among some plants unusually high temperatures not only 

 are tolerated but also actually accelerate development. Seeds of the 

 wattlebark tree are commonly boiled 1 hour to hasten germination 

 before planting. The seed cones of certain species of pine trees ( e.g., 

 Pinus hanksiana) open promptly only after fire has scorched them. 

 This fact has obvious implications for the ecological effects of forest 

 fires as will be discussed later. 



Another special adaptation for dealing with extremes of temperature 

 is dormancy. The term hihernation is often used loosely to describe 

 all instances in which metabolism is reduced during winter when the 

 environment becomes too cold. A great many poikilothermous ani- 

 mals go into such a hibernating condition in crevices, under rocks, or 

 in the mud. Plants similarly are sometimes spoken of as hibernating 



