7 o PHYSIOLOGY OF STREAMING MOVEMENTS 



were exposed to light. According to Borscow and Luerssen 1 this action 

 of light is a general phenomenon, the red rays exercising the greatest 

 effect and causing a retardation and ultimate cessation of streaming. This 

 result is, however, owing to the heating effect of the red rays, and if the 

 preparations are kept cool, it is easy to see that the blue rays are most 

 effective (in the presence of oxygen). On the other hand, other authors 2 

 could detect no influence of light on streaming. This was probably owing 

 to the use of light of feeble intensity, for Pringsheim 3 has shown that 

 exposure to intense light causes a pronounced retardation and rapid 

 cessation of streaming in Nitella, Tradescantia, and Spirogyra, while the 

 author has extended these observations to Chara, Elodea, Vallisneria, as 

 well as to Hydra viridis and Vorticella campanula 4 . 



The intensity of the illumination necessary is usually very much over- 

 estimated. For example, sunlight passed through cold alum solution, and 

 concentrated by a lens to a photochemical intensity of about eight times 

 that of bright direct sunlight, causes a cessation of streaming in Elodea 

 leaves within six minutes, while if the alum solution is removed, a complete 

 temporary or permanent stoppage is produced in from one to four minutes. 

 Sections of leaves of Vallisneria are even more sensitive, but Chara is 

 usually less so. Thus five minutes' exposure may produce complete rigor 

 on the margins of leaves of Elodea, whereas eight minutes' similar exposure 

 may be required to produce the same effect on end-cells of Chara. In 

 the latter case it can usually be seen that after the first twenty or thirty 

 seconds streaming is accelerated, sometimes to twice the original speed, 

 then slowing rapidly after three to five minutes, and ceasing or becoming 

 very slow by the time the chloroplastids are bleached. When slow 

 streaming is still present at the end of a period of exposure, it may go 

 on slowing until it permanently ceases, or may gradually acquire its 

 original velocity. The first acceleration is probably due to the heating 

 effect, which can never be entirely eliminated, and it is always more marked 

 if the preparation is in a cool medium to commence with. If the pre- 

 paration is previously wanned no acceleration can usually be detected, 

 but this observation is of little value, for whenever streaming is at, or 

 nearly at, its maximal velocity, a new stimulus must either exercise no 

 appreciable effect or must cause a retardation. 



Six to eight hours' continuous exposure to bright direct sunlight 

 suffices to make streaming cease or become very slow in Chara and Elodea, 

 while under weak illumination it is usually fairly active again in a quarter of 



1 Borscow, Bull. d. 1'acad. de St.-Petersbourg, 1868, XII ; Luerssen, Einfluss des rothen.und blauen 

 Lichtes auf die Stromung des Protoplasmas, 1868. 



2 Cf. Hauptfleisch, Jahrb. f. wiss. Bot., 1892, Vol. XXIV, p. 173. 



3 Jahrb. f. wiss. Bot., 1882, Bd. XII, pp. 326-44. 



4 Ewart, Ann. of Bot., 1898, Vol. xn, pp. 383-90. 



