INFLUENCE OF EXTERNAL CONDITIONS UPON PERIODICITY 209 



tance, and in fact seems to induce a certain periodicity in the algae of the 

 Mediterranean and North Sea. Although complete rest does not actually take 

 place, many algae vegetate during summer, and, produce reproductive organs 

 in winter 1 . 



SECTION 60. The Influence of the External Conditions upon the Yearly 



Periodicity. 



Metabolism and respiration, though they never entirely cease, appear 

 to be considerably depressed during the resting period 2 , even when the 

 plant is under favourable conditions. A wound may cause an increased 

 respiration and a formation of callus and cork during an autogenic resting 

 period 3 , although the wound-reaction takes place more slowly than when 

 the plant is in full activity, and does not seem able to awaken the plant 

 permanently from its dormant condition. Similarly cuttings taken from 

 a plant during its resting phase develop roots slowly and with difficulty, 

 while the buds may often be unable to expand. 



A low temperature during winter or a diminution of turgor decreases 

 the activity of respiration and also increases the length of the resting period. 

 Furthermore the removal of the spring foliage causes in many trees an im- 

 mediate development of buds which would have gradually enlarged during 

 summer into resting winter-buds. In this case the slow development of 

 summer and the resting period of winter are omitted. This is also the case 

 when a tree under unusual conditions flowers a second time in autumn, 

 owing to the direct development of some of the resting buds 4 . The resting 

 period may be shortened in many plants by treatment with anaesthetics, by 

 changes of temperature, or by transitory desiccation. Other agencies may 

 produce similar effects, and may even cause an entire omission of the 

 resting period without changing any of the plant's hereditary properties or 

 its inherent periodicity. 



Johannsen 5 found that treatment with ether or chloroform for twelve to twenty- 

 four hours caused the buds of such plants as Syringa vulgaris and Prunus triloba to 

 open from three to six weeks earlier than untreated plants, whereas in other cases 

 the resting period was shortened but little or not at all. Not only growth but 



1 Berthold, Mittheil. a. d. Zool. Station zu Neapel, 1882, Bd. in, p. 429; Oltmanns, Jahrb. 

 f. wiss. Bot., 1892, Bd. xxin, p. 413; Kuckuck, Ber. d. Bot. Ges., 1897, p. 446; Schimper, 

 Pflanzengeographie, 1898, p. 446. 



2 Miiller-Thurgau, Landw. Jahrb., 1885, Bd. XIV, p. 861 ; N. J. C. Miiller, Beitr. z. wiss. Bot. 

 von Fiinfstiick, 1898, II, p. 247. 



8 Rechinger, Verb. d. Zool.-Bot. Ges. in Wien, 1893, p. 317; Jost, Bot. Ztg., 1893, p. 100; 

 Wakker, Bot. Centralbl., 1887, Bd. XXXII, p. 239. 



* See Mobius, Beitrage zur Lehre von der Fortpflanzung, 1897, p. 105. Injuries may cause 

 potatoes to sprout earlier than usual. Cf. Jost, 1. c., p. 101. 



5 Johannsen, Bot. Centralbl., 1898, Bd. LXVin, p. 337; Das Aetherverfahren beim 

 Friihtreiben, 1900. 



PFEFFER. II P 



