THE PHOTIC ORIENTATION OF CHLOROPLASTIDS 333 



than an hour from the night to the day position, whereas in less readily responsive 

 plants a few hours may be required. As is generally the case, the new position 

 is assumed more rapidly than the original position is regained when the stimulus is 

 removed. In plants which react rapidly enough, the chloroplastids therefore undergo 

 daily changes of position. Such parts as the thallus of Marchantia, in which light 

 causes the chloroplastids to collect upon the outer walls, will assume a deeper green 

 when illuminated l . The continued paling shown by many plants in prolonged dark- 

 ness is, however, in part the result of pathological changes in the chloroplastids, 

 coupled with a decomposition of the chlorophyll. The shadow figures produced by 

 Sachs 2 by partially covering leaves with tinfoil or black paper are not solely the result 

 of the primary photic reaction. Similarly, the paling of leaves in intense light 3 

 though in the first instance partly due to movements of the chloroplastids, is mainly 

 the result of the partial decomposition of the chlorophyll 4 . 



Historical. The changes of position of the chloroplastids in the leaves of 

 Crassulaceae, when exposed to sunlight, were discovered by Bohm 5 , and the details of 

 the process, as well as the change of the reaction with increasing intensity of illumina- 

 tion, were investigated by Famintzin, Borodin, and especially by Frank 6 . Frank 

 considered the changes of position to be due to diffuse phototonic stimulation, whereas 

 Stahl 7 considers them to be phototactic orienting responses to the direction of the 

 illumination. Many undoubted instances of phototactic or phototropic orientation are 

 given by Stahl, but diffuse actions may also be exercised, as was shown by Schimper 

 and Haberlandt 8 . Frank 9 used the term ' epistrophe ' to indicate the normal orienta- 

 tion of the chloroplastids in light, and ' apostrophe ' for that assumed in darkness, or 

 owing to the action of other factors. Schimper 10 , and also Moore n , used Frank's terms 

 in a slightly different sense, and distinguished the balling together of the chloro- 

 plastids as ' systrophe.' None of these terms are, however, really necessary, since 

 light-position, dark-position, superficial, lateral, and aggregated positions indicate all 

 the possible movements in the cell, and profile and face positions, flattened and con- 

 vex shapes, describe those of the chloroplastid. 



1 See Stahl, Bot. Ztg., 1880, p. 329 ; Schimper, 1. c., p. 225 ; Moore, 1. c., p. 233. First 

 observed by Borodin and Frank. 



2 Sachs, Sitzungsb. d. Sachs. Ges. d. Wiss., 1859, p. 226; Stahl, 1. c. 



3 First observed by Marquart, Die Farben d. Bliithen, 1835, p. 47. 



4 Pringsheim, Pringsh. Jahrb. , 1879-81, Bd. XII, p. 374; Keeble, Annals of Botany, 1895, 

 Vol. IX, p. 63; Ewart, Annals of Botany, 1898, Vol. XII, p. 384. 



5 Bohm, Sitzungsb. d. Wiener Akad., 1856, Bd. XXII, p. 479; 1853, Bd. xxxvil, p. 453. 



6 Famintzin, Jahrb. f. wiss. Bot., 1867-8, Bd. vi, p. 45 ; Borodin, Bull, de 1'Acad. de St. Peters- 

 bourg, 1867, T. IV, p. 482 ; Melanges biologiques de St. Petersbourg, 1869, Bd. vn, p. 50 ; Frank, 

 Jahrb. f. wiss. Bot, 1872, Bd. VIII, p. 216. 



7 Stahl, Bot. Ztg., 1880, p. 297. 



8 Schimper, Jahrb. f. wiss. Bot., 1885, Bd. xvi, p. 203; Haberlandt, Ber. d. bot. Ges., 1886, 

 p. 209. 



9 Frank, 1. c., p. 221. 1} Schimper, 1. c. 

 11 Moore, Journ. Linn. Soc., 1888, Vol. XXIV, p. 200. 



