236 THE POWER OF RESISTANCE TO EXTREMES 



different organs on the same plant, according to their specific character l and 

 stage of development. A good instance of the latter is afforded by the 

 winter buds of trees, which are highly resistant, although the expanding 

 leaves and flowers are readily killed by frost. Soaked seeds are somewhat 

 more resistant than the seedlings 2 . Seeds exposed on the surface will be 

 dry and hence fully resistant, while moist ones will be partially protected 

 by the layers of soil covering them. 



As the result of adaptation, most tropical plants are readily killed by 

 cold, although some are actually more resistant than certain temperate 

 plants or than particular organs adapted only for growth during summer 3 . 

 Plants with a low minimum for growth are not always especially resistant 

 to cold, for the swarm spores of Ulothrix zonata are killed by freezing at 

 1 C. 4 , although the minimum temperature for growth and for their 

 movement lies at o C. It has yet to be determined whether those marine 

 algae are especially resistant which grow at 1-8 C. but are not naturally 

 cooled appreciably below this. 



The resistance to cold depends to a certain extent upon the present 

 and previous external conditions. Thus Haberlandt 5 found that seedlings 

 grown at 1 8 to 20 C. froze more readily than those grown at 8 C. Goppert 6 

 observed that Senecio vttlgaris, Poa annua, and Fiimaria officinalis^ which 

 had been exposed to temperatures as low as 11 C. during November and 

 December, were killed by freezing at 9 C. after they had been in a warm 

 house for a fortnight. In etiolated plants the power of resistance seems 

 also to be decreased. It is probably in adaptation with their habitat that 

 plants grown in alpine regions are somewhat more resistant to cold than 

 individuals of the same species grown in the plains beneath. 



Unfavourable conditions appear in general to reduce the power of 

 resistance to cold, and a deficiency or excess of water or of a food-material 

 acts in this way 7 . A variety of other factors may also enter into play, 

 and hence it arises that Coniferae with a north exposure are usually more, 

 but occasionally less, resistant than those facing south. 



The influence of the withdrawal of water. It appears as though objects 



1 The roots are usually less resistant than stems (Mohl, Bot. Ztg., 1848, p. 6; 1862, p. 324; 

 Goppert, Ueberdas Gefrieren und Erfrieren d. Pflanze, 1883, p. 56). 



3 Gb'ppert, Die Warmeentwickelung in der Pflanze, 1830, p. 43 ; G. Haberlandt, Die Schutzein- 

 richtungen d. Keimpflanze, 1877, P- 4^- 



z Cf. Karsten, Bot. Ztg., 1861, p. 289; Goppert, Bot. Jahresb., 1873, p. 263; Naudin, Ann. d. 

 sci. nat, 1877, 6 e sen, T. v, p. 323. 



* Strasburger, Wirkung d. Lichtes u. d. Warme auf Schwarmsporen, 1878, p. 62. 



6 G. Haberlandt, Die Schutzeinrichtungen d. Keimpflanze, 1877, P- 4 8 5 Bot - Jahresb., 1879, 

 p. 259. 



6 Goppert, 1. c., 1830, p. 63. 



7 Cf. Miiller-Thurgau, Landw. Jahrb., 1886, Bd. XV, p. 543. Also Duhamel, Naturgesch. d. 

 Baume, 1765, Bd. II, p. 270. 



