THE PHYSICAL BASIS OF LIFE 201 
It will be convenient now to turn our attention to the 
microscopic examination of a plant that is sufficiently trans- 
parent to enable us to look within its living parts and observe 
the behavior of protoplasm. The first thing that strikes one 
is the continual activity of the living substance within the 
boundaries of a particular cell. This movement sometimes 
Fic. 81.—(A) Rotation of Protoplasm in the Cells of Nitella. 
(B) Highly Magnified Cell of a Tradescantia Plant, Showing 
Circulation of Protoplasm. (After Sedgwick and Wilson.) 
takes the form of rotation around the walls of the cell (Fig. 
81 A). In other instances the protoplasm marks out for itself 
new paths, giving a more complicated motion, called circula- 
tion (Fig. 81 B). These movements are the result of chemi- 
cal changes taking place within the protoplasm, and they are 
usually to be observed in any plant or animal organism. 
Under the most favorable conditions these movements, as 
seen under the microscope, make a perfect torrent of un- 
ceasing activity, and introduce us to one of the wonderful 
sights of which students of biology have so many. Huxley 
