82 INFLUENCE OF THE EXTERNAL CONDITIONS ON GROWTH 



equable temperature appears to act as a stimulus upon Vanchcria and 

 Oedogonium, inducing a formation of swarm-spores l . A frequent repetition 

 of such changes of temperature very possibly may shorten the normal 

 resting periods. Here, and in other cases also, it is less the stimulating 

 action of the mere change, than the direct action of the new temperature 

 which is of importance. It is not quite certain whether the disturbance 

 of growth observed by Schreiber -, when spores of Bacillus antJiracis 

 which had germinated at 38 C. were brought to 18 C., was due to the 

 direct action of the lower temperature, or to the stimulating action of the 

 change. 



In nature plants are subjected to sudden as well as to gradual 

 changes of temperature, so that the existence of a power of rapid 

 accommodation is of great importance. This does not, however, exclude 

 the possibility of a special power of reaction to changes of temperature 

 being developed for particular purposes. The rate of growth of each 

 organ is proportional to its temperature, and the general vital activity 

 is not disturbed by the temporary adaptation of the various parts to 

 slightly different temperatures 3 . If, however, certain organs are kept 

 permanently at injurious extremes of temperature, the other organs are 

 also ultimately affected owing to the interdependence of the various parts. 



It is not known why growth should decrease beyond a certain 

 temperature, although respiration continues to increase and molecular 

 movements in general become more rapid. The temperature curve of 

 growth is, moreover, by no means always the same, and it is even possible 

 that two maxima may be shown. So far as is known, the curve at first 

 ascends slowly from the minimum, but then more rapidly. As it approaches 

 the optimum it again ascends more slowly, and beyond the optimum it falls 

 slowly or rapidly according to the nearness or remoteness of the maximum 

 from the optimum. 



History. After Lefebvre 4 had determined the maximum and minimum 

 for the germination of Raphanus, these extremes and also the optimum were 

 determined for a variety of seedlings by de Candolle 3 , as well as by Edwards 

 and Colin 6 . The whole subject was then closely investigated by Sachs 7 , and 

 many authors have subsequently carried out researches upon seedlings, as well 

 as with lower organisms. 



Methods. The cardinal points can only be approximately determined, and 



1 Klebs, Bedingungen d. Fortpflanzung, 1896, pp. 49, 269. No such effect was observed in 

 Hydrurus. 



2 Schreiber, Centralbl. f. Bact., 1896, Bd. XX, p. 372. 



3 Godlewski, Anzeig. d. Akad. d. \Viss. z. Krakau, 1890, p. 172. 



4 Lefebvre, Exper. s. 1. germination, 1801, p. 124. 



5 De Candolle, Pflanzenphysiologie, 1835, Bd. II. 



6 Edwards and Colin, Ann. d. sci. nat., 1834, 2 e se>., T. I, p. 270; 1836, 2 e sen, T. V, p. 7. 



7 Sachs, Jahrb. f. wiss. Bot., 1860, Bd. II, p. 338. 



