CIRCULATION AND ROTATION OF PROTOPLASM. 617 



directly perceptible by means of the microscope: moreover it need not be simply 

 in an alteration in the circulation and rotation that the irritability of the proto- 

 plasm to gravitation and light has to express itself; and in the subsequent con- 

 sideration of the movements of the chlorophyll-corpuscles we shall see that these 

 are probably to be referred to the stimulation of colourless protoplasm by light. 



Heat acts very energetically as a stimulus, and this so that the streaming 

 movement is accelerated as the temperature increases up to an optimum. Tem- 

 peratures beyond this optimum, again, retard the movement, and finally (at about 

 45° C.) the filamentous network of circulating protoplasm contracts into a clump; 

 however, if this action has not continued too long, the normal configuration 

 of the protoplasm may be restored after a short time at a lower and more 

 favourable temperature S Strong electric shocks act similarly, and it is here to, 

 be especially insisted upon that pressure on the cell-wall occasionally induces; 

 similar effects, and in some cases at least a complete arrest of the movement : 

 subsequently it returns. Hence it happens that in recently made preparations of 

 hairs or internal cells abounding in protoplasm, the circulation and rotation are not 

 observable at all during the first few minutes, or it may be for some time, whereas the. 

 same preparations often exhibit vigorous movement a few hours later. According to 

 Dehnecke's recent observations, moreover, lying in water for some time, and there- 

 fore probably a certain diseased state of the protoplasm, acts so as to accelerate 

 the movements, so that they are often much more vigorous a long time, after the 

 preparation was made than in the normal condition^. It should be mentioned 

 here that the rotating and circulating protoplasm continues its movements even 

 when the water of the cell-sap is affected exosmotically by the action of sugar- 

 solution, the protoplasm separating from the cell-wall on all sides and contracting ; 

 in fact in the case of the root-hairs of Hydrocharis it is sometimes even possible 

 to break in pieces the contracted protoplasm within the cellrwall, and each of the 

 pieces forms a thick-walled vesicle of protoplasm, the substance of which continues 

 to rotate actively for some time longer. 



So long ago as 1861 I had called attention to the remarkable fact that the 

 leaves of the most various plants appear bright green in very intense sun-light, 

 and dark green in the shade, and that it is possible accordingly to obtain a sort 

 of light-picture by laying a strip of black paper on a leaf on which the sun is 

 shining : on removing the paper the shaded part appears dark green, the parts 

 exposed to the light bright green. I did not at that time succeed in obtaining 

 the right explanation of this fact, but now, from later researches by Famintzin,. 

 Frank, Borodin, and especially by Stahl, it has been discovered : they showed that, 

 the chlorophyll-corpuscles under the influence of the light assume different positions 

 within the cells, and this necessarily gives to the whole leaf the appearance described 

 above. Since these phenomena pave the way to still more general points of view 

 and accurate ideas as to heliotropism proper, it is worth while to enter into 



' Details on Ihese matters are found in my treatise, ' Vber die obere Temperatur-grenze der 

 Vegetation,' Flora, 1864 (p. 37). Velten, •Einwirkung der Temferatur auf Protoplasma-bewegung' 

 Flora, 1876 (Nos. 12-14). 



^ Dehnecke, in 'Flora,' 1881 (Nos. i and 2). 



