96 RELATION OF PLANTS TO TEMPERATURE 



drawal of the water of constitution from the plasma and forma- 

 tion of it into crystals either inside or outside the cell wall. In the 

 latter case the formation of intercellular ice crystals may often 

 result in tearing apart the cells of the tissues, but in no case has 

 it been found to rupture the walls except in very violent lowering 

 of the temperature. Low temperatures even above the freezing 

 point may be fatal to an organism. A few plants such as Agave 

 Americana may be frozen, and if the temperature is not carried 

 too low, may revive if thawed slowly. In most instances how- 

 ever the rapidity of thawing from ordinary low temperatures is 

 without bearing upon the fatality of the process as also thawing 

 in air or water. The rate at which the temperatures of plants 

 exposed to liquid air or liquid hydrogen (— 252° C.) is raised is 

 of importance on account of the great physical disturbances in- 

 volved. Not all of the tissues of a plant are equally resistant to 

 cold. The embryonic elements probably succumb most easily, 

 while the stomatal and trichome cells are most resistant in the 

 vegetative body. The most effectual adaptations for the en- 

 durance of low temperatures are to be found in reproductive 

 bodies of all kinds. Some very interesting observations may 

 be made upon the effects of low temperatures upon plants 

 if the student will spend the day following the first heavy 

 frosts in the field examining the native species for frost reactions. 

 A list of the species affected, and the organs killed should be 

 made. It will be seen that perennial plants have varying pro- 

 portions of their bodies killed by frost. In trees only the leaves 

 may die ; in shrubs and shrubby plants the shoot may die down 

 to the ground, and in certain herbaceous forms all but some 

 thickened fleshy roots may perish. 



The topography of the region should also be taken into account 

 and the accumulation of cold air in valleys be followed and the 

 effects noted. 



150. Formative Effect : Thermal Constants. The formative ef- 

 fect of temperature is scarcely differentiated so far as its influence 

 upon individuals are concerned, although the general adaptations- 



