208 PHYSIOLOGICAL BOTANY. PART II. 



to clear away the outer tubes which are always more 

 or less encrusted with carbonate of lime, in order to 

 expose the inner tube in which the rotation of the 

 fluid may be seen. This is an operation requiring some 

 little delicacy ; and the choice of a 

 species of the other section (Nitella) is 

 to be preferred, in which the tubes are 

 generally very transparent and require 

 no preliminary preparation to clean their 

 surface. At the joints of the stem are 

 whorls of branches (fig. 158.) com- 

 posed also of short tubes, in each of 

 which the same rotation of the con- 

 tained fluid may be seen. If an entire 

 tube occupying the space between two 

 joints be detached and placed under the microscope, 

 its inner surface appears to be studded with minute 

 green granules arranged in lines, which do not run 

 parallel to the axis of the tube but wind in a spiral 

 direction from one extremity to the other. They are 

 studded over the whole of the interior, with the exception 

 of two narrow spaces on opposite sides of the tube form- 

 ing two spiral lines from end to end. The globules of 

 transparent gelatinous matter dispersed through the fluid 

 are in constant motion, being directed by a current up 

 one side of the tube and back again by the other. The 

 course of this current is regulated by the spiral arrange- 

 ment of the granules, and it moves in opposite directions 

 on contrary sides of the clear spaces on the inner surface of 

 the tube. The rotation continues in a detached portion, 

 for several days ; and if the tube is tied at intervals 

 between the joints the fluid between two ligatures still 

 continues to circulate, even though the extremities of the 

 tube should be cut away. The motion here described 

 is precisely similar to what takes place in the tubes of 

 Corallines, and must unquestionably be considered as 

 the result of a vital action. 



(195.) Local Circulation*. It was in the year 

 1820, that a distinguished naturalist, M. Schultes, 



