THE PHOTIC ORIENTATION OF CHLOROPLASTIDS 329 



disturbances which have no relation to the direction of the light, and which 

 may also be produced by temperature extremes as well as by injuries and 

 various mechanical agencies 1 . On the other hand, the orienting action of 

 ordinary light is well shown by the fact that oblique illumination may 

 cause the chloroplastids to collect at the opposite corners of the cells in the 

 unilamellar leaf of a Moss 2 . In addition, the chloroplastids in Bryopsis 

 move towards the better illuminated portion of the cell 3 . 



Even when we are dealing with simple tissues and responsive chloro- 

 plastids, the positions assumed do not always correspond precisely to the 

 above rules, partly owing to the shape of the cells and their relationships to 

 neighbouring ones. The position of the chloroplastids in the palisade-cells 

 of leaves is but little or not at all influenced by light, and the same applies 

 to other cells as well. This may either be due to the absence of any 

 phototactic irritability or to the suppression of any aitiogenic response by 



FlG. 55. Mesophyll-cells from the under-surface of the leaf of Oxalis acetosella seen from above: (a) face 

 position of chloroplastids in diffuse light; (b) profile position after short exposure to sunlight ; (c) after longer inso- 

 lation (after Stahl). 



more powerful aitiogenic factors. In addition, the light- rays may be con- 

 centrated by refraction or reflection upon particular areas, as in the proto- 

 nema of Schizostega (Schistostegd), or the light may be so dispersed that a 

 cell or cells in the interior may be uniformly illuminated on all sides even 

 when the leaf is under unilateral external illumination. 



In the tissues of many of the higher plants the choroplastids show 

 similar groupings in response to light as do those in the leaf-cells of Mosses. 

 Thus in the spongy mesophyll of Oxalis acetosella the chloroplastids arrange 

 themselves upon the walls parallel to the surface in diffuse light (Fig. 55 tf), 

 whereas in direct sunlight a profile position is assumed (Fig. 55 b\ and after 

 long exposure the balling together shown in Fig. 55*: takes place. In the 

 leaf-cells of Sempervivum and Sedum y according to Stahl (I.e.), the face 

 position is assumed in shade, an intermediate position in bright diffuse 

 light, and an aggregated position in sunlight. 



1 Cf. Frank, 1. c., pp. 261, 295 ; G. Kraus, Bot. Ztg., 1874, p. 206; Haberlandt, Ueber d. 

 Einfluss d. Frostes auf die Chlorophyllkorner, 1876, p. 6 (reprint from the Oester. Bot. Zeitschrift) ; 

 Schimper, 1. c., pp. 166, 235 ; Moore, 1. c., pp. 206, 371. 



3 Stahl, 1. c., p. 346. 



8 Winkler, Jahrb. f. wiss. Bot., 1900, Bd. xxxv, p. 455. 



