THE PHOTIC ORIENTATION OF CHLOROPLASTIDS 331 



makes use of the locomotory energy of the surrounding cytoplasm or whether changes 

 of surface-tension of its own production are responsible for its movement. The 

 chloroplastids may indeed possess no power of photic reaction at all and may be 

 carried to or from the light by the cytoplasm, but this can hardly apply to the orien- 

 tation of the chlorophyll-bands of Mougeotia or of the chloroplastids of Vaucheria and 

 of Moss-leaves. The nucleus, for instance, does not accompany the movement of the 

 chloroplastids, and it is only in a few cases that a certain accumulation of protoplasm is 

 shown where they collect 1 . It remains, however, to be determined whether the phototac- 

 tic chloroplastids merely act as directive agencies or are directly responsible for their 

 own movement. The ' active ' view supported by Velten 2 and Stahl s is as devoid of 

 proof as is that according to which the movement is passive (Frank *, Moore 5 , and 

 Oltmanns 6 ). In any case, isolated chloroplastids show no power of orientation how- 

 ever long they may remain living and capable of photosynthesis, and in whatever 

 media, apart from the cytoplasm, they may be placed. Nor do dead chloroplastids 

 show any phototactic orientation within a living cell 7 . 



Although the orientation of the chlorophyll-band of Mougeotia and of the chloro- 

 plastids of Vaucheria and Mosses appears to be due to the direction of the light-rays, 

 it is possible that in other cases the movements may be produced in response to 

 changes in the general intensity of diffuse illumination. Haberlandt 8 considers the 

 latter to be the case in Moss-leaves and Fern-prothallia, but the evidence given above 

 points rather to the opposite conclusion. It must be remembered that rays falling 

 perpendicularly to the surface of a Moss-cell mostly penetrate, whereas those with an 

 oblique incidence are mostly reflected. Hence a directive action may be exercised 

 even in diffuse light or when the plant is rotated on a klinostat. The condition is the 

 same as when a cylinder containing zoospores is rotated in diffuse light, but has strips 

 of partially opaque paper pasted on its sides. A phototactic response will be -shown 

 towards the better illuminated areas in feeble light, but away from them when the 

 light is intense. 



Within the tissues where the light is dispersed in all directions, responses may be 

 more commonly produced by changes in the intensity of the illumination; and in fact 

 when very intense illumination produces changes of position and a balling together of 

 the chloroplastids, this is not the result of any tropic stimulation. When moderately 

 strongly illuminated on one side the chloroplastids of Vaucheria, Moss-leaves, and 

 Fern-prothallia collect on the opposite sides of the cell. This cannot be due to their 

 possessing dissimilar powers of reaction, since they retain their original position when 

 the shaded under-surface of a prothallus is exposed to light 9 . The chloroplastids 

 are, therefore, not dorsiventral, and in fact they appear to respond as a whole and 

 not individually to changes in the direction and intensity of the illumination. The 

 chlorophyll plate of Mougeotia is also isobilateral but diaphototropic. 



1 Frank, Jahrb. f. wiss. Bot., 1872, Bd. VIII, p. 283. 



2 Velten, Aktiv oder passiv? Oester. Bot. Zeitschr., 1876, No. 3. 



3 Stahl, Bot. Ztg., 1880, p. 351. Frank, 1. c., p. 282. 



5 Moore, Journ. Linn. Soc., 1888, Vol. xxiv, pp. 203, 264. Oltmanns, Flora, 1892, p. 211. 



7 Ewart, Protoplasmic Streaming in Plants, 1903, p. 108. 



8 Haberlandt, Physiol. Pflanzenanat., 2. Aufl., 1896, p. 234. 9 Stahl, 1. c., p. 350. 



