sect, ii PHYSIOLOGY 245 



protoplasm directly contiguous to the cell wall is not in motion. The 

 rotatory movements are easily seen in Cham and NiteVa, where they 

 take a spiral course, and they are also very energetic in the cells of the 

 leaves of Elodea canadensis and of Fallisneria spiralis, and also in the 

 root-hairs of Hydrocharis mmszis ranae and Trianea bogotensis. 



The cause of these movements, which may take different directions in adjoining 

 cells, and may also continue after the protoplasm has heen drawn away from the 

 cell walls hy plasmolysis (p. 166), is not yet understood. It is, however, known 

 that the continuance and activity of such protoplasmic movements, the existence of 

 which was first observed hy Coeti in 1772, and later rediscovered by Trevieanus 

 in 1807, are dependent on factors which, in general, support and promote the vital 

 activity ; while the presence of free oxygen and proper conditions of temperature 

 seem to be particularly favourable to them. Through the study of sections in the 

 cells of which currents had been induced in the protoplasm, by the injuries sustained 

 in their preparation and hy other abnormal conditions, grave errors have arisen 

 concerning the existence of such protoplasmic movements in cells, in which under 

 normal conditions they cannot be observed. The presence of protoplasmic currents 

 in a cell may, in fact, indicate either an energetic vital activity, or, on the other 

 hand, be merely a symptom of a pathological or, at least, of an excited condition of 

 the protoplasm. 



The movements of orientation of the protoplasmic body do not 

 proceed in the same uninterrupted manner as the circulatory and 

 rotatory movements. They are also usually so gradual as to be only 

 recognisable through their operations. They are induced by changes 

 in the external influences, especially as regards the intensity of the 

 light, and result in producing a definite position of the protoplasmic 

 bodies, as, for example, the orientation of the chlorophyll grains with 

 regard to the light. 



Movements of this kind have been most frequently observed in Algae, in sub- 

 merged Duckweed (Zemna trisulca), in the prothallia of Ferns and Mosses ; hut 

 similar movements can also be observed in the higher plants. 



In the cells of the filamentous Alga Mesocarpus, the chloroplasts, in 

 the form of a single plate suspended length-wise in each cell, turn 

 upon their longitudinal axes according to the direction and intensity 

 of the light. In light of moderate intensity, according to Stahl's 

 observations, they place themselves transversely to the source of light, 

 so that they are fully illuminated (transverse position) ; when, on the 

 other hand, they are exposed to direct sunlight, the chlorophyll plates 

 are so turned that their edges are directed towards the source of light 

 (profile position). A similar protection of the chloroplasts against too 

 intense light, and their direct exposure, on the other hand, to more 

 moderate illumination, is accomplished, where they are of a different 

 form and more numerous, by their different disposition relative to the 

 cell walls. In moderate light the chlorophyll bodies are crowded 

 along the walls, which are transverse to the direction of the rays of 



