224 THE POWER OF RESISTANCE TO EXTREMES 



The power of resistance varies considerably according to the plant examined, 

 and hence the conditions for existence are not in all cases alike. Palms and other 

 tropical plants cannot be grown in the open in our climate, whereas our indigenous 

 trees can withstand cold during their winter resting condition, although frost kills 

 the expanded leaves and flowers. The aerial parts of many plants with perennating 

 rhizomes are killed by frost, as are also many annuals whose seeds perennate over 

 winter. Similarly plants which are sensitive to desiccation maintain themselves 

 over periods of drought by seeds, spores, rhizomes, bulbs, tubers, and the like. 

 If the entire plant is to survive it must either possess xerophytic adaptations to 

 guard against an excessive loss of water, as in the case of cactuses and many 

 Euphorbias, or it must be able to withstand desiccation, as in mosses and lichens. 



The injurious actions of cold, heat, light, and desiccation are constantly met 

 with in nature, but the influence of poisons also requires discussion, since this 

 is a subject which is acquiring increasing physiological importance. Indeed it is 

 possible by the use of anaesthetics and other poisons to retard or inhibit certain 

 functions, while others are affected but little or not at all. We are not, however, 

 concerned with plant pathology *, and indeed diseases are not always accompanied 

 with visible formative changes, which naturally can hardly occur when death is 

 instantaneous. Signs of death are, the drooping, discolouration, shrivelling of 

 herbaceous parts 2 , the changed appearance of the protoplasm, its non-plasmo- 

 lysability, its absorption of stains, and its permeability to soluble pigments present 

 in the cell-sap 3 . The appearance of the protoplasm differs, however, according 

 to the mode of death, and by means of picric or osmic acids, or of absolute alcohol, 

 it may be fixed in its original condition 4 . Otherwise the structure is more or less 

 effaced after death, but such post-mortem changes have no physiological importance, 

 and the same applies to the changes which may be produced in the cell-wall by 

 the energetic action of certain reagents. 



PART II 



RESISTANCE TO HEAT AND COLD 

 SECTION 65. Supra-maximal Temperatures. 



The position of the ultra-maximal temperature beyond which death 

 ensues varies not only in different plants, but also according to the stage 

 of development and the external conditions. Thus seeds and plants which 



suitable conditions, and hence is only conditionally immortal. Cf. Verworn, Allg. Physiol., 1897, 

 2. Aufl., p. 345. 



1 Frank, Krankheiten d. Pflanzen, 1895, 2. Aufl. ; Hartig, Lehrb. d. Baumkrankheiten, 

 1899, 2. Aufl. ; Sorauer, Handb. d. Pflanzenkrankheiten, 1886, 2. Aufl. 



3 Cf. Sachs, Flora, 1864, p. 37; Hofmeister, Pflanzenzelle, 1867, P* IO > ^e Vries, Sur 1. mort. 

 d. cellul. ve"ge"tales, 1871 (repr. from Archives Neerlandaises, T. vi). 



3 The protoplasm may be killed without the diosmotic properties of the plasmatic membrane 

 being at first lost. 



* Fischer, Fixirung, Farbung u. Bau d. Protoplasmas, 1899. 



