SOME PROPERTIES OF PROTOPLASMIC GELS : I. TENSION IN THE CHLOROPLAST OF SPIROGYRA

The chloroplast of Spirogyra is a long, spirally coiled ribbon which may contract to form a short, nearly straight rod. This happens under natural conditions and it can also be produced by a variety of inorganic salts and by some organic substances. It also occurs when the chloroplast is freed by centrifugal force from the clear peripheral protoplasm which is in contact with the cellulose wall. It would therefore seem that the chloroplast may be passively stretched by the action of the clear protoplasm and hence it contracts as soon as it is set free. This contraction happens in dead as well as in living cells. It would be of much interest to know how the protoplasm brings about the coiling of the chloroplast and how the chloroplast is set free by various reagents. Presumably they must penetrate the living protoplasm to produce the effects described. In one species partial contraction without detachment from the peripheral protoplasm can be brought about by lead acetate. This is reversible. Lead nitrate does not produce this result. The attack upon the problem is greatly facilitated by the study of dead cells. Thereby we reduce the number of variables but the chloroplast continues to react to certain chemical and physical agents in much the same manner as in the living cell and the solution surrounding it can be controlled as is not possible in the living cell. We must await further investigation to learn what plant and animal cells contain gels under tension and what functions they perform.