220 THE POWER OF RESISTANCE TO EXTREMES 



mosses and lichens the entire plant can withstand desiccation, whereas among 

 Phanerogams and fungi only the seeds or spores possess this power. The 

 power of resistance may change during development, for the winter buds of 

 our forest trees survive frosts which may readily kill the young foliage and 

 expanding buds. It may also be modified by the previous conditions, and 

 is dependent upon the duration and rapidity of application of the injurious 

 agency. The effect of an intermittent application is, moreover, not the same 

 as that of a constant one. The resistance of seeds, spores, and mosses 

 against cold, heat, and poisons is greatly increased by drying, and the action 

 of one poison may be neutralized by that of another. 



A striking instance of the effect of sudden change is afforded by the 

 bursting of fungi when a concentrated solution in which they are growing 

 is suddenly diluted, whereas gradual dilution to the same extent does not 

 injure them. Similarly a plant may slowly accommodate itself to strains, 

 concentrations, doses of poison, and temperatures which would injure or kill 

 it when suddenly applied. A repeated change may produce a cumulative 

 effect, as when a plant which can withstand a single frost is killed by 

 successively freezing and thawing a few times. This is possibly due to 

 a gradual weakening of the organism and a consequent decrease in its 

 power of resistance. 



The intensity of any agency required to produce a fatal effect depends 

 not only upon its duration, but also upon many variable circumstances. 

 Hence the position of the ultra-maximum l , as well as of the maximum and 

 minimum points, can only be approximately and conditionally determined. 

 The same applies to the ultra-minimum, which is non-existent as regards 

 water or heat when a plant is uninjured by low temperatures or by desicca- 

 tion, or when we are dealing with a non-essential agency. 



In turgid plants a complete cessation of all forms of vital activity 

 involves irremediable death, for even when growth, movement, and photo- 

 synthesis have ceased, respiration still continues so long as the plant is 

 living and ceases only on its death. In fact, the plant is usually no longer 

 capable of recovery when its respiratory activity has been brought to 

 a minimum or even much reduced by the action of an injurious agency 2 . 



Even when a plant is frozen, respiration is merely strongly reduced, 

 and probably ceases only when all the free and imbibed water passes into the 

 solid condition, which may require temperatures of 12 C. and even lower 3 . 

 Perfectly dry seeds, mosses, and lichens do not respire, and neither 



1 This term was introduced by Engelmann, Handbuch d. Physiol. von Hermann, 1879, Bd. I, 



P. 358. 



2 In starved fungi respiration may be reduced to a very low ebb, whereas at supra-maximal 

 temperatures a slight diminution of the original respiratory activity indicates a permanent injury. 



3 Koch, Biol. Centralbl., 1890, Bd. x, p. 682 ; de Candolle, Archives d. sci. phys. et nat. 

 d. Geneve, 1895, T. xxxni, p. 506. Cf. also van Tieghem and Bonnier, Bull. d. 1. Soc. Bot. d. 

 France, 1882, T. xxix. 



