210 PERIODICITY OF GROWTH 



also respiration and metabolism are affected, and since submaximal doses of poisons 

 may produce an increased respiration and production of heat, it may also be possible 

 to shorten the resting period by means of them. 



Since the result depends upon the condition of the plant, it is not surprising 

 that little or no effect can be produced in summer or early autumn, when the growth 

 and activity of the buds is at a minimum \ The fact that the buds may be brought 

 to immediate development in spring by removing the foliage shows that their power 

 of response does not trace a curve parallel to that of the grand period. 



Temperature. Knight observed in 1801 that a vine exposed to transitory cold 

 sprouted sooner than those kept permanently in a greenhouse. Pfeffer obtained the 

 same result with Ampelopsis, Syringa, and Lydum^ Miiller-Thurgau with potato- 

 tubers 2 . The resting period may be shortened in this way to a variable degree under 

 natural conditions, but it has not yet been determined whether a single prolonged 

 exposure to cold or repeated changes are more effective, or whether the stimulus is 

 stronger with or without freezing. Metabolism is actually altered in the potato and 

 other plants by temperatures even above o C. Possibly in some plants the resting 

 period may be shortened by temporary exposure to high temperatures 3 . 



Desiccation awakens the resting stages of Chlamidomonas*, and considerably 

 hastens the germination of the zygotes of Chlorogonium 5 and of the seeds of 

 Eichhornia and Heteranthera 6 . Transitory drying seems to accelerate the sprouting 

 of the bulbs of Hyacinthus and Tulipa, and of the corms of Crocus. The degree 

 and duration of desiccation required to produce the maximal effect have not, 

 however, been accurately determined 7 , nor is it certain whether Braun and Miiller 

 are correct in assuming that previous drying is essential for the further development 

 of CklamidomonaS) or for the germination of the seeds mentioned. 



A precise determination of the factors concerned in these alterations of 

 periodicity is rendered more difficult because of the influence of the previous 

 conditions upon the tone of the plant and hence upon its response. Such 

 inductive action may persist for a long time and may only gradually dis- 

 appear in the course of several generations. Thus the ripening of the seeds 

 of cereals and of other plants is hastened in northern regions, and when 

 such seeds are grown in warmer climates the life-cycle is at first shorter than 

 usual, but as the plant adapts itself, the life-cycle lengthens to the normal 



1 Cf. also Jost, Bot. Ztg., 1891, p. 605 ; Lutz, Beitr. z. wiss. Bot. von Fiinfstiick, 1895, Bd. I, 

 p. 78. 



2 Mtiller-Thurgau, Landw. Jahrb., 1882, Bd. xi, p. 816; 1885, Bd. xiv, p. 903. Eriksson 

 (Centralbl. f. Back, 1895, 2. Abth., Bd. I, p. 557) finds that transitory exposure to low temperatures 

 favours the germination of the spores of certain Aecidiomycetes. 



3 Cf. Wiesner, Biol. d. Pflanzen, 1889, p. 47. 



* Braun, Betracht. iiber die Erscheinung d. Verjiingung in der Natur, 1850, p. 228. 



5 Klebs, Unters. a. d. Bot. Inst. zu Tubingen, 1883, Bd. I, p. 340. Cf. Schroder, ibid., 1886, 

 Bd. n, p. 24. 



6 Fr. Miiller, Biol. Centralbl., 1886, Bd. VI, p. 299. Cf. also Batalin, Bot. Centralbl., 1889, 

 Bd. xxvin, p. 706. 



7 Schroder, 1. c. 





