IMBIBITION THEORIES 271 



he had also observed so-called centrifugal currents moving 

 in the opposite direction, "which would agree with the 

 contraction hypothesis. He thus saw himself forced to 

 assume causes of two kinds to explain the two streams ; 

 the former would arise by an " expansion " of the peripheral 

 protoplasm at the edge of the plasmodium, the latter, on 

 the contrary, by a contraction of it. 



Hofmeister (1865, 1367) and many recent investigators 

 have seen the streams in plasmodia and Amcebse always arise 

 at the forwardly moving edge. For my own part I drew 

 attention to this fact in 1873 for the large Amceba terricola, 

 and for this reason declared myself opposed to the contrac- 

 tion theory. Hofmeister (1865 and 1867) was also able 

 to convince himself that in the hair cells of Tradescantia 

 the currents extend from before backwards. 



This observation caused him to turn against the con- 

 traction theory just as Nageli and Schwendener (1865) had 

 also done. The latter authors remarked with reference to 

 protoplasmic streamings in plant cells that contractility 

 really explained nothing, the more so as this conception 

 was obscure and unintelligible for protoplasmic bodies. 

 Under such circumstances streamings could only arise 

 according to the analogy of a blood circulation. We see 

 also, as a matter of fact, that the further development of 

 the contraction theory did necessitate this idea. 



(e) Hypotheses of Hofmeister, Sachs, aTid Engelmann 



Hofmeister, therefore, attempted to found a special 

 hypothesis of protoplasmic movements, and in particular of 

 the protoplasmic streaming in plant cells, the identity of 

 which with those of Ehizopods was recognised on all sides. 

 He thought (1865) that these phenomena of streaming 

 might be referred to the very varying power of imbibition 

 of the protoplasm, which was chiefly manifested in the play 

 of the contractile vacuoles. With regard to the contractile 

 vacuoles, however, he held erroneous ideas. The individual 

 " particles " of protoplasm were supposed to possess a very 

 different and varying power of imbibition. If this power 



