330 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS 



As the result of the dispersal of the penetrating rays, a tropic orienta- 

 tion of the chloroplastids will be less useful in the interior of a tissue 

 than it is on the surface. Nevertheless it is at the surface where the 

 light is strongest that protective movements are of most importance, 

 while in feeble light the position the superficial chloroplastids assume 

 still further darkens the interior of the tissue, especially in thick leaves. 

 This is one of the reasons for the thinness of shade leaves, since in this way 

 they acquire greater adaptability to varying intensities of illumination. All 

 chloroplastids do not, however, show a power of photic orientation, even when 

 in superficial cells. This power is in fact absent from the cells of Cladophora, 

 Nitella> Spirogyra> and from most palisade parenchyma cells. In these the 

 accumulation of the chloroplastids at the upper ends of the cells would 

 hinder instead of favouring the utilization of the light rays, which is best 

 performed when they lie on the side walls. Stahl * and Haberlandt 2 have, 

 however, shown that light exercises a strong orienting action upon the 

 chloroplastids in the palisade-cells of many plants, which Haberlandt found 

 to be especially pronounced when the light was at right angles to the long 

 axis of a palisade-cell exposed by sectionizing. 



A certain advantage is attained by the fact that the immovable 

 chloroplastids of palisade-cells bulge inwards during moderate illumination, 

 but flatten themselves against the wall when the light is intense. This was 

 first discovered by Micheli 3 , while Stahl observed the following changes of 

 shape of the chloroplastids of Ricinus in shade and sunlight : 



In this way the chloroplastids are less exposed to intense illumination, 

 but intercept more light when the latter is less intense. These and other 

 changes of shapes are not restricted to the chloroplastids of palisade-cells, 

 but are shown in others also, and may not only be produced by intense 

 illumination but also by various other agencies, such as continued darkness, 

 extremes of temperature, saline solutions, and chemical substances 4 . 



Neither the mode of stimulation nor the mechanism of movement of the chloro- 

 plastids is as yet known, apart from the fact that chloroplastids may be passively 

 carried by streaming protoplasm. It is even uncertain whether the chloroplastid 



1 Stahl, Bot. Ztg., 1880, p. 377. 



2 Haberlandt, Physiol. Pflanzenanat., 2. Aufl., 1896, p. 210. 



3 Micheli, Arch. d. sci. de la Bibl. univers. de Geneve, 1876, T. xxix, p. 26; Stahl, 1. c., p. 357. 

 * Stahl, 1. c., 1880, pp. 303, 361; Schmitz, 1. c., 1882, p. 82 ; Berthold, Jahrb. f. wiss. Bot., 



1882, Bd. xin, p. 691 ; Klebs, Unters. a. d. bot. Inst. zu Tubingen, 1883, Bd. I, p. 268; 1886, Bd. n, 

 P- 557 J Schimper, 1. c., p. 240 ; Moore, 1. c., p. 643 ; Haberlandt, Flora, 1888, p. 296 ; de Vries, 

 Ber. d. bot. Ges., 1889, P- J 95 Tswett, Bot. Centralbl., 1897, Bd. LXXII, p. 329; Kolkwitz, Fest- 

 schrift f. Schwendener, 1899, p. 271. 



