MOVEMENTS OF CHLOROPLASTS 679 



shade leaf of Fagus, as compared to a sun leaf of the same species. The two 

 lowest curves in figure 22.14 indicate a sightly larger difference between the 

 spectral transmission curves of a dark-green and a light-green Hibiscus 

 leaf. Finally, figure 22.10 illustrates the effect of extreme variations in 

 chlorophyll content, such as occur in aurea leaves. Here, a few per cent 

 of the normal chlorophyll content (cf. Table 15.1, Vol. I) suffice to produce 

 from two thirds to nine tenths of normal absorption in the region between 

 520 and 700 m^. 



Without varying the concentration of the pigments, many plants have 

 it in their power to adjust the light absorption by the displacement or re- 

 orientation of the chloroplasts. These tactic reactions, discovered by Bohm 

 in 1856, were investigated by Stahl (1880, 1909), Senn (1908, 1909, 1917, 

 1919), Liese (1922) and Voerkel (1933), among others. It was found that 

 each chloroplast moves independently, i. e., it is not carried by streaming 

 of the protoplasm. In moderate light, the chloroplasts gather on the illumi- 

 nated front walls and orient themselves so as to present their large cross- 



1 I 



Apostrophe Antistrophe Diastrophe Porastrophe 



Fig. 22.4. Schematic representation of different chloroplast orientations (after 

 Benecke and Jost 1924). Arrows show direction of light incidence. 



sections to the light ("antistrophe," "epistrophe" and "diastrophe" in 

 figure 22.4; the first one is produced by one-sided, and the other two by 

 two-sided, illumination). In strong direct light, on the other hand, the 

 chloroplasts turn their axes parallel to the light beams and line the side 

 walls of the cells ("parastrophe" in fig. 22.4). During the night, they often 

 assume characteristic "night positions" — congregate around the nuclei, or 

 disperse throughout the cytoplasm, or line the internal walls ("apostrophe" 

 in fig. 22.4). The largest variety of chloroplast movements has been ob- 

 served by Senn in green and brown algae, and in diatoms. In leaves, the 

 chloroplasts in the parenchyma cells assume positions similar to those 

 shown in figure 22.4, but the chloroplasts in the palisade tissue usually re- 

 main arrayed along the side walls, leaving the end walls free. Instead of 

 moving bodily these chloroplasts merely change their shape : In strong dif- 

 fuse light, they spread flat against the walls, whereas in weak light they 

 protrude into the cytoplasm, without losing contact with the walls. lUu- 



