THE PHYSICAL BASIS OF LIFE 



261 



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 





Fig. 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 



