THE CARDINAL POINTS FOR GROWTH 79 



The alga of red snow (Sphacrella nivalis) also grows at a low 

 temperature, but it is not certain whether, like the Arctic algae, it can 

 grow actively when the temperature is permanently maintained at or below 

 o C. Such plants as Hclleborus nigcr, Croats, Tussilago Farfara, and 

 Soldanella grow and flower in winter or spring, and in Ficaria rannncu- 

 loides the aerial organs die down in summer after a period of active vegeta- 

 tion in spring just as they do in Hydrurus. 



The temperature does not always lie within the limits for growth, 

 and in all such cases the plant, if not in itself sufficiently resistant, must 

 perennate over the unfavourable period by means of special vegetative 

 organs, by seeds, or by spores. Almost all the flowering plants of 

 temperate regions perennate in some form or other through at least one 

 period of the year, and such plants as Ulothrix and Hydrums, whose 

 vegetative parts are killed by a temperature of 20-24 C., perennate during 

 summer in the form of spores. 



Organisms which grow under extremes of temperature are subject 

 to very little competition, and hence we should expect that all readily 

 disseminated organisms capable of withstanding temperatures over 50 C. 

 would ultimately find their way to a hot spring. The fact that each 

 thermophile organism has its own temperature-limit shows that the power 

 of adaptability is not in all cases the same, and that this power is not 

 capable of indefinite extension under the action of regularly recurring 

 conditions of heat or cold. It is, however, not impossible that organisms 

 may be able to exist or grow at temperatures approaching 100 C., although 

 development could hardly occur if the organism were completely frozen. 

 The latter, however, only takes place in small organisms at temperatures 

 several degrees below o C. 



The cardinal points can never be determined with more than approxi- 

 mate accuracy, since their position is influenced by the external conditions, 

 by the duration of the exposure, by the age of the plant, and by its 

 previous treatment. Growth may in fact continue for a time at tempera- 

 tures which ultimately bring about its cessation or even the death of the 

 plant. Hence growth can never be aroused at such temperatures, although 

 at more equable ones the growth of a partially injured plant may slowly 

 be resumed again 1 . 



The influence of the external conditions on the cardinal points. Brefeld 2 

 observed that the sporophore of Coprinus stercorarius was formed in light at 



1 Kirchner, Cohn's Beitr. z. Biol., 1883, Bd. in, p. 362 ; Askenasy, Ber. d. Bot. Ges., 1890, p. 75 ; 

 Pfeffer, Druck- u. Arbeitsleistung, 1893, p. 354; True, Annals of Bot., 1895, Vol. IX, p. 387; 

 Hilbrig, Ueber d. Einfluss supramaximaler Temperatur auf das Wachsthum, Leipziger Dissertation, 

 1900. 



3 Brefeld, Bot. Unters. ii. Schimmelpilze, 1877, In > P- 935 Griintz, Einflnss d. Lichtes a. d. 

 Eutwickelung einiger Pilze, Leipziger Dissertation, 1898, p. 29. 



