PROTOPLASMIC STREAMING—BERTHOLD 291 



local alterations of the surface tension between the fluid 

 protoplasm and the cell-sap. "Weber in 1855 had already- 

 pointed out the similarity between the streamings observed 

 in certain drops, and produced by surface tension, and the 

 protoplasmic streamings in plant cells. It is without doubt 

 a great merit on the part of Berthold to have recognised 

 correctly the importance of these relations, and to have 

 ventured an attempt to follow them up consistently. Even 

 in 1865 Nageli and Schwendener had very rightly declared 

 that the true seat of the motor forces in the streamings of 

 plant cells must be the surface of the protoplasm turned 

 towards the cell-sap ; on the other hand, they formed an in- 

 correct judgment upon the state of affairs, inasmuch as they 

 considered that this movement of the surface of the proto- 

 plasmic body obtained its point of support in the surrounding 

 water, " just in the same way as a bird in the air or a fish in 

 the water," and that in this way the forward movement took 

 place. They concluded, therefore, that the adjacent cell-sap 

 must always be in a condition of streaming "in an opposite 

 sense to " the movement of the protoplasm. This erroneous 

 idea, founded upon incorrect views as to thp nature of the 

 streamings, was especially contradicted by Velten (1873), 

 who showed that the movement of the adjacent cell-sap, as 

 far as can be judged from the fine granules occasionally 

 occurring in it, always took place m the same sense as that 

 of the protoplasm. From these undisputed observations it 

 also follows with certainty that the streaming of the proto- 

 plasm reaches as far as its limit on the side of the cell-sap, 

 a fact which may also frequently be directly observed, so 

 that no resting skin-like layer exists at this spot. It seems 

 to me quite impossible to assume with Wakker (1888), 

 who also observed this movement in the cell-sap, that the 

 protoplasmic streaming sets the membranous layer lining the 

 cell-sap vacuole or cavity in rotation by means of its fric- 

 tion ; especially if we consider that in cell-sap cavities, which 

 are spanned by bridges of protoplasm, such displacements 

 of the vacuole would necessarily be very obvious. 



According to Berthold's hypothesis it is a case of differ- 

 ences in the surface tension, at the limit between ceU-sap 



