THE PLANT AND THE ANIMAL 253 



a delicate section with a sharp razor of a leaf or stem . 

 of a water plant, such as Vallisneria/ so thin as to be 

 transparent, being at the same time careful not to 

 injure the uncut cells. Water plants are convenient 

 simply because microscopic investigations are generally 

 made in water, which means that the cell remains in 

 its natural medium. If all the natural conditions are 

 fulfilled, i.e. if the temperature is not too low and the 

 cells are not injured, one of the most curious phenomena 

 ever presented by the organic world reveals itself in a 

 short time to our eyes. The cell-sap, or rather that part 

 of the cell-contents which we have called protoplasm,^ 

 and which as a layer of thick liquid lines the inner 

 surface of the cell-walls, or stretches in strands across 

 the cavity of the cell, which is filled with a thinner sap, 

 — this protoplasm moves in every cell, slowly at first, 

 then more and more quickly. This movement is seen 

 especially clearly in cases (such as Vallisneria) where 

 bright green chlorophyll grains are suspended in the 

 protoplasm. These grains, carried by the rapid current 

 of the protoplasm can be seen flowing along one of the 

 longitudinal cell -walls, turning along the transverse 

 wall to the other longitudinal wall, and from it again 

 along the second transverse wall, to come back to the 

 point of departure and repeat over and over again that 

 circular voyage. This rapid rotating movement of 

 the protoplasm can be observed in one and the same 

 cell for hours and even days. In cells in which the 

 protoplasm forms a general network of strands, the 

 movement is not limited to the circular current along 

 the walls, but is also seen to flow in thin streams across 

 the cavity of the cell. Such a movement can be 

 observed in any hair, such as the familiar hairs of 



1 A plant to be found in any aquarium. The curious phenomena taking 

 place in this plant during the period of pollination have been described 

 in the previous chapter (fig. 68) . 



2 See chapter ii. 



