34 PHYSICS OF STREAMING 



life of the cell. According to Berthold l t the plane of rotation is always 

 constant, being parallel to the surface of the leaf in Vallisneria, and at right 

 angles to the surface in the cortical cells of Chara. The plane of rotation 

 can, however, be artificially altered by the death of neighbouring cells, by 

 localized injuries (heat, intense illumination) to particular cells, and by 

 exposing cells which have been kept in darkness for some time to strong 

 light (Elodea}. Similarly, a change in the direction of streaming can some- 

 times be observed in cells of Vallisneria and Elodea after the application 

 of stimuli sufficiently powerful to cause a permanent stoppage and death in 

 some cells, and a temporary stoppage in the rest. No such reversal ever 

 seems to occur in Chara or Nitella, although it is possible, by producing 

 localized light or heat rigor at the middle of a cell, to break the stream into 

 two halves circulating in the unaffected ends. 



According to Berthold (I.e., p. 122) there is no constant relation 

 between the direction of streaming in contiguous cells of Vallisneria and 

 Elodea. This is an error, for almost without exception, the direction of 

 streaming is opposed on the two sides of each dividing wall, i.e. is either 

 with or against the hands of a watch in all the cells of a leaf. 



A curious type of streaming sometimes occurs. The plasma streams 

 along a single large strand crossing the vacuole, and, spreading out like 

 a fountain jet, returns along the opposite side walls of the cell. It occurs 

 in hairs of Cucurbita, young endosperm cells of Ceratophyllum (Velten), 

 and in the young segments of the wood vessels of Ricinus (de Vries) 2 . 



The central strand may be connected to the peripheral layers by thin 

 lateral strands, and this type may pass into ordinary circulation. 



Alex Braun 3 showed that the order of development of lateral leaves 

 and roots in Chara bore a definite relation to the direction of streaming in 

 the parts from which they rise, those to which the stream is directed 

 developing first. This is probably largely a question of nutrition, for there 

 can be no doubt that streaming in the large cells of Chara and Nitella is of 

 the utmost importance for rapid translocation. Indeed, Hermann 4 con- 

 cludes that the spiral streaming in the larger cells of Chara and Nitella is 

 an adaptation to favour translocation. That lateral diffusion to and from 

 the medullary cells of Chara may be accelerated by the spiral direction 

 of the streaming layers in the cortical cells is obvious, but it is not easy 

 to see how longitudinal transference can be accelerated by the adoption 

 of a longer path. It is impossible, moreover, to follow Hormann in his 

 attempts to deduce physiological conclusions, unsupported by experiment, 

 from morphological facts. 



1 Protoplasmamechanik, p. 122. 



2 Velten, Bot. Ztg., 1872, p. 651 ; de Vries, Bot. Ztg., 1885, p. 22. 

 8 Konigl. Akad. d. Wiss., Berlin, 1852. 



* Protoplasmastromung bei den Characeen, Jena, 1898, p. 13. 



