TROPIC ACTION OF LIGHT ON FREELY MOTILE ORGANISMS 327 



in plant-cells kept in light which had passed through potassium bichromate 

 solution was probably due to some accessory heating or other effect. 

 Reinke and Kraus l were in fact unable to detect any such deformations in 

 the protoplasm of epidermal hairs under similar exposure. 



SECTION 70. The Photic Orientation of Chloroplastids. 



The movements produced by the action of light serve not only to bring 

 the chloroplastids into suitable functional positions, but also to withdraw 

 them from the action of intense light. Other agencies such as temperature, 

 chemical actions, and the withdrawal of water may also affect the position, 

 and autogenic alterations of the normal position are also possible 2 . In 

 addition, when active streaming is excited by an injury to the leaves of 

 Vallisneria and Elodea, the chloroplastids may be carried with the plasma 

 for a variable length of time until the resting condition is again assumed 3 . 

 During normal streaming either none or only occasional chloroplastids are 

 carried with the streaming protoplasm. Pringsheim found 4 that the chloro- 

 plastids bleached by sunlight in cells of Nitella were carried away by the 

 streaming endoplasm, whereas in cells of CJiara they 

 retain their original positions 5 , and the most varied 

 agencies fail to cause them to leave the ectoplasm in 

 Nitella*. Nevertheless, slight disturbances of position 

 are probably easily produced, and these are very pro- 

 nounced in such Diatoms as Rhipidophora and Striatella, 

 for mechanical vibrations cause their chloroplasts to 

 retract and become spherical 7 . 



A phototropic orientation is especially evident in 

 the chlorophyll plates of Moitgeotia and Mesocarpus, 

 which under favourable conditions place themselves at 

 right angles to the incident rays (Fig. 53 A), but in strong fLng ]?ght. P s 

 light twist round until a profile position is assumed with 

 the flat surface parallel to the direction of the light 8 . In other cases, 





FIG. 53. Cells of Meso- 

 carpus illuminated from 

 beneath. A, face posi- 

 tion in moderate light ; 



rothen u. blauen Lichtes auf die Stromung d. Protoplasmas, 1868. Cf. also Velten, Die physikal. 

 Beschaffenheit d. pflanzl. Protoplasmas, 1876, p. 14 (reprint from the Sitzungsb. d. Wiener Akad., 

 1876, Bd. LXXIII, Abth. i) ; Famintzin, Jahrb. f. wiss. Bot., 1867-8, Bd. VI, p. 38. 



1 Reinke, Bot. Ztg., 1871, p. 800; G. Kraus, Bot. Ztg., 1876, p. 584. 



2 See Fr. Schmitz, Die Chromatophoren, 1882 ; Schimper, Jahrb. f. wiss. Bot., 1885, Bd. xvi, 

 p. 203 ; Hauptfleisch, 1. c. ; Haberlandt, Physiol. Pflanzenanat., 2. Aufl., 1896, p. 232. 



3 Hauptfleisch, Jahrb. f. wiss. Bot., 1892, Bd. xxiv, p. 192. 

 * Pringsheim, Jahrb. f. wiss. Bot., 1879, Bd - XII > P- 333- 



5 Ewart, Journ. Linn. Soc., 1897, Vol. XXXI, p. 574. 



6 Pfeffer, Jahrb. f. wiss. Bot., 1900, Bd. xxxv, p. 723. 



7 Liiders, Bot. Ztg., 1862, p. 42 ; Schmitz, Chromatophoren, 1882, p. 82 ; Schimper, I.e., p. 218. 



8 Stahl, Bot. Ztg., iSSo, p. 299; Moore, Journ. of Linn. Soc., 1888, Vol. xxiv, p. 366; 



