358 CHLOROPLASTS AND CHROMOPLASTS CHAP. 14 



5 X 10^; the corresponding figure for Mnium, according to Godnev and 

 Kalishevich, was 9 X 10^. 



The chloroplasts have been diversely described as liquid or solid 

 systems. They are more adequately defined as thixotropic gels — see, for 

 example, Menke (1938) — that is, solid colloids which can be liquefied by 

 weak mechanical forces. This temporary liquefaction enables the 

 chloroplasts to change their shape, grow pseudopodia, propagate by 

 division, and occasionally discharge their contents through holes in the 

 cell walls. 



The existence of a chloroplast membrane was suggested by Tschirch 

 (1884), who stated that it prevents chloroplasts from coalescing, and 

 protects chlorophyll from being destroyed by organic acids present in 

 the sap of many plants. Others, for instance, Schmitz (1883), considered 

 the membrane an optical illusion. More recently, Wieler (1936) re- 

 affirmed the existence of a chloroplast membrane in Elodea canadensis, 

 and Granick (1938^ stated that a semipermeable membrane permits 

 maintaining isolated chloroplasts of tobacco and tomato intact for several 

 hours in a 0.5 molar glucose solution. When isolated chloroplasts are 

 placed in distilled water, they swell, become vacuolated, and disintegrate 

 (cf. Neish 1939). 



2. The Grana 



Earlier investigators, e. g. Pringsheim (1881), Schmitz (1883), Tschirch 

 (1884), and Bredow (1891), stated that chloroplasts have a structure 

 which they described as a "sponge" (Pringsheim) or "net" (Schmitz). 

 Meyer (1883) and Schimper (1885) called the structure "granular," 

 with dark "grana" surrounded by a lighter colored "stroma." Then 

 came a change in opinion. Liebaldt (1913) and Ponomarev (1914) 

 described chloroplasts as homogeneous bodies ; this interpretation received 

 strong support from the then prevalent concept of the structure of the 

 living protoplasm, which was considered as an homogeneous colloidal 

 system — hydrogel or hydrosol — without microscopic differentiation. All 

 structural details, often observed in the protoplasm under the microscope, 

 were supposed to be artefacts, indicating a denaturation of the living 

 matter. This point of .view was extended to chloroplasts; and in all 

 textbooks up to 1935 they were pictured as homogeneous, optically 

 empty, colloidal bodies. Zirkle (192(3) asserted that chloroplasts often 

 contain a vacuole, connected by channels with the cytoplasm, and that 

 these channels can give the chloroplast an apparent granular structure 

 which may have deceived earlier observers. 



In 1932, however, the hypothesis of chlorophyll grana was revived by 

 Heitz (1932). It was confirmed by photographs (Doutreligne 1935) clearly 

 showing the dark grana among lighter-colored "stroma" in the chloro- 



