SPECIAL AND GENERAL 153 



while a deficiency of oxygen may render proper ripening difficult or impossible, so 

 that if all free oxygen is removed while the sporangia or zoospores are unripe no 

 dispersal or dehiscence takes place l . 



Apart from the physical action of temperature upon imbibition and the like, 

 a physiological action is also exercised upon the development preparatory to 

 dehiscence and dispersal. Plants adapted to low temperatures are able to throw 

 off organs and to discharge their swarm-spores or other reproductive bodies at 

 temperatures approaching the freezing-point of water or even slightly below it, 

 especially in the case of Arctic marine Algae 2 . Certain observations of Thuret 

 seem to indicate that the escape of the zoospores is delayed at temperatures above 

 the optimum, while in some cases changes of temperature appear to accelerate the 

 escape. Thus, Dodel observed a premature birth of the zoospores of Ulothrix when 

 frozen filaments of this Alga were rapidly thawed. 



Light appears to exercise little or no direct physical influence upon these 

 movements, for when it accelerates transpiration or induces the development of 

 reacting organs, or of a reacting condition, its action is as indirect as when 

 illumination causes movement by modifying the growth or turgor of responsive 

 cells 3 . 



The dehiscence and dispersal movements of ripe organs may take place in 

 temporary darkness even when the organs are unable to develop or do not develop 

 normally in continued darkness. Illumination or changes of illumination do, however, 

 appear in certain cases to favour these movements. Thus the illumination of previously 

 darkened plants hastens the throwing off of the sporangia of Pilobolus crystallinus * 

 and the ejaculation of the spores of Ascobalusfurfuraceus^. In addition, light appears 

 to favour the escape of the swarm-spores of many Algae, and in darkness the 

 zoosporangia may not be as completely emptied, or their contents as well dispersed, 

 as when illuminated 6 . 



1 Cf. Rothert, Cohn's Beitrage z. Biologic, 1892, Bd. V, p. 344, and the literature quoted by him. 



2 For instances see Kjellmann, Bot. Ztg., 1875, p. 774; G. Kraus, ibid., 1875, p. 774; Dodel, 

 ibid., 1876, p. 178 ; Strasburger, \Virkung des Lichts und der \Varme auf Schwarmsporen, 1878, p. 44 ; 

 Klebs, Die Bedingungen der Fortpflanzung einiger Algen und Pilze, 1896. 



3 [The implied suggestion that the physical action of light is always a direct one, and its 

 physiological action indirect, is somewhat misleading. Possibly the only direct physical action of 

 light is the mechanical pressure exercised upon an illuminated surface by the impinging light-rays. 

 The chemical, heating, and fluorescent effects of light are as much indirect actions as when illumina- 

 tion affects turgor or transpiration, and in each case the percentage of the light energy utilized 

 depends upon the properties of the material affected.] 



* According to Coemans and to Hofmeister, Pflanzenzelle, 1867, p. 290. G. Kraus (Bot. Ztg., 

 1876, p. 507) states that the blue and violet rays are most effective. 



5 Coemans, quoted by de Bary, Morphologic und Biologic der Pilze, 1884, p. 99. 



6 For the literature see Braun, Verjiingung, 1851, p. 237 ; Thuret, Ann. sci. nat., 1850, 3" ser., 

 T. xiv, p. 247; Strasburger, I.e., p. 15; Walz, Bot. Ztg., 1868, p. 497; Dodel-Port, ibid., 1876, 

 p. 177; Rostafinski n. Woronin, ibid., 1877, p. 667; Klebs, I.e. 



