30 Plant Physiology 



rapid movement. It is a phenomenon so common 1 

 that it must be assumed to have physiological significance. 

 Moreover, it is often rapid in cells of large size, so that 

 it seems safe to say that it is not unimportant in facili- 

 tating diffusion. A study of this capacity for movement 

 gives an impressive mental picture of the protoplasm as 

 the seat of activity in the cell or member. 



The various types of protoplasmic movement are com- 

 monly grouped in four categories : (1) simple streaming, 

 (2) circulation, (3) rotation, and (4) orientation. 



Streaming movements are rather spasmodic in different 

 parts of the cell, first in one direction, and sooner or later 

 a reversal. Aside from the slime molds, the coenocytic 

 filaments of the black molds show, among plants, pro- 

 nounced movements of this type. 



Circulation consists in movement at any instant in 

 more than one direction in the cell. The motion may 

 occur in the peripheral cytoplasm, but this type is char- 

 acteristic of cells possessing cytoplasmic strands. In 

 fact, when the cytoplasmic strands are lost, the movement 

 may become rotary. Circulation may be conveniently 

 observed in the stamen hairs of Tradescantia and in the 

 stem and leaf hairs of various plants, especially cucurbits; 

 also in young root-hairs and other young cells. 



Rotation, or the movement in a rather constant current 

 or direction around the cell, or in some area of the cell, is 

 the most striking type. It usually occurs in cells which 



1 For a list of greenhouse material suitable for the study of movement 

 the following paper may be consulted : Bushee, Grace L., The Occur- 

 rence and Rate of Protoplasmic Streaming in Greenhouse Plants. Botan. 

 Gaz., 46 : 50-53, 1908. 



