RELATION BETWEEN STREAMING AND ASSIMILATION 55 



46 C. streaming ceases in five to ten minutes, and at 42 C. in fifteen to 

 thirty minutes, in both cases recommencing in five to fifteen minutes at 

 20 C. Preparations kept at 40 C. to 42 C. for an hour may show no 

 recommencement of streaming, however long they are exposed to light, 

 although streaming may reappear if oxygen is admitted, and may or may 

 not continue when the preparations are closed again, according to whether 

 the power of photosynthesis has been permanently or only temporarily lost. 

 A rise of temperature accelerates streaming and respiration, but above 

 certain limits retards or inhibits photosynthesis. Hence in closed cell- 

 preparations, a rise of temperature to over 40 C. at first accelerates stream- 

 ing, which later slows and ultimately ceases in most cases. It may then 

 recommence some time after the temperature has been lowered to 20 C. if 

 the illumination is maintained, while, if free oxygen is admitted, it begins at 

 once, or almost at once, in all cells where it was previously present. 



SECTION 22. Relation between Growth and Streaming. 



Very young growing cells exhibit for some time after division has 

 ceased, only very slow and irregular protoplasmic movements. It is only 

 as the cell grows older that active streaming becomes possible, and regular 

 rotation begins only when the cell is fully grown or nearly so. Hence 

 a certain antagonism seems to exist between the two functions, although 

 the growth of young cells in which ' secondary streaming ' has been induced 

 by artificial excitation does not seem to be retarded in any way. 



In the case of large and especially of long cells (Ckara, Nitella, midrib 

 cells of Elodea, &c.) the continuance of streaming seems to form an essen- 

 tial condition of existence. When once commenced, it never ceases under 

 normal conditions, and it cannot be stopped permanently, or for at all 

 prolonged periods of time, without injuriously or fatally affecting the vitality 

 of the cell. This is probably because in such cells the circulation of the 

 protoplasm is an important factor in regulating its nutrition, and hence 

 becomes an ingrained habit. External growth is, however, always the 

 direct result of the activity of the non-streaming ectoplasm, the rotating 

 endoplasm securing a rapid transmission of food-materials from one part to 

 another. Hence even where ' primary ' streaming exists there is no direct 

 but frequently a marked indirect connexion between it and external growth. 



SECTION 23. The Influence of the Nucleus on Protoplasmic Movement. 



The retention of the power of movement by non-nucleated fragments 

 of animal cells is a well-established fact. Thus Hofer 1 has shown that 

 under such conditions the contractile vacuoles of Infusoria may continue to 



Exp. Unters. iiber den Einfluss d. Kernes auf das Protoplasma, 1889, p. 486. 



