52 THE ROYAL SOCIETY OF CANADA 



observation. When the cells are plasmolysed it is common to find 

 some whose chloroplasts are too rigid to contract with the cytoplasm 

 but maintain their position more or less, while the cytoplasm is 

 pulled in between the coils. The spiral form itself I take to be an 

 expression of elasticity, being the position of minimum curvature, 

 and so of least resistance, for an elastic body confined in a closed 

 cylinder shorter than itself. Thus in species whose chloroplasts are 

 no longer than the cell they are straight and longitudinal, the more 

 their length exceeds that of the cell the closer the spiral. From all 

 these considerations it would seem that under ordinary conditions the 

 physical state of the chloroplasts is that of an elastic gel.* 



The changes in form.- — The "contraction" which is observed in 

 abnormal chloroplasts is not a simple phenomenon but is of two very 

 different types. In type I the body behaves as a liquid and the 

 contraction is manifestly an endeavour to assume minimal area. 

 The sinuosities of margin first smooth out, the whole chloroplast 

 thickens and shortens, becomes cylindrical and finally spherical, or 

 else constricts into several spheres. During the longitudinal con- 

 traction the coils may slip round the plasmatic layer if the spiral is 

 loose; if it is tight they usually contract across the vacuole to one 

 side of the cell, pulling films of protoplasm with them. If more than 

 one chloroplast is present they may become tightly twisted like a 

 rope before they have time to uncoil. The phenomenon is not always 

 so simple of explanation — contraction may begin before the margin 

 smoothes out (see later). The second type resembles the first inas- 

 much as there is longitudinal contraction with somewhat similar 

 results, but instead of thickening as it contracts the chloroplast 

 becomes thinner, its margin becomes more sharply toothed than 

 normally and vacuoles may develop around it. Progress toward 

 minimal area does not take place. The appearance suggests a 

 syncretic contraction of the gel, with expulsion of water. Changes 

 in other cell structures usually accompany the above. Some of the 

 cytoplasmic strands that support the nucleus in its median position 

 give way, and the latter is pulled to one side of the cell by contraction 

 of the other strands. Diminution of viscosity giving free play to the 

 action of surface tension would explain this as well as the first type of 

 chloroplastic change, but that the syncretic type of contraction may 

 also take place is shown by occasional vacuolization and other evi- 

 dence to be described. With certain factors (heat, etc.) the contraction 

 of the cytoplasmic strands appears to pull the chloroplasts toward the 

 nucleus. The same two types of behaviour are shown by the chloro- 

 plasts of other plants. Mougeotia resembles Spirogyra, allowing for 



