CHAP. XL] SPIROGYRA. 397 



distinguish. The most favourable for investigation are of 

 course the larger ones, and among these the most con- 

 venient forms are those in which the spiral bands have 

 somewhat lax windings, so that they do not interfere with 

 the view into the interior of the cell. 



Each cell of Spirogyra is bounded on the outside by a 

 cellulose wall. This consists of a number of layers, which 

 however can only be made out distinctly by the use of 

 reagents such as potash. In many species the wall is sur- 

 rounded by a thick gelatinous sheath. Immediately inside 

 the cell-wall and closely applied to it is a layer of proto- 

 plasm, to which the name primordial utricle is applied. In 

 Spirogyra this is so thin that it is difficult to make out under 

 natural conditions except with a very high power. It can 

 however be easily demonstrated by the use of what are 

 termed plasmolysing re-agents, such as solution of common 

 salt, which withdraw water from the interior of the cell, and 

 thus cause its contents to contract. The contracted mass 

 will then be seen to be bounded on all sides by a thin 

 membrane, which is the primordial utricle. The latter is of 

 precisely the same nature as the protoplasm in the cells of 

 Yeast or Protococcus. Its occurrence in the form of a 

 relatively thin parietal layer, while the greater part of the 

 cavity of the cell is occupied by a large vacuole, is character- 

 istic of most vegetable cells in their mature condition. 



The green spiral bands correspond to the chromatophores 

 in Protococcus, and bear the same name, each band being 

 spoken of as a single chromatophore. Each cell may there- 

 fore contain one chromatophore, or several, according to 

 the species. In these bodies all the chlorophyll of the 

 plant is contained. They themselves consist of protoplasm 

 and retain their form unaltered when the chlorophyll is 

 extracted by alcohol or other solvents, and they are in im- 

 mediate contact with the primordial utricle throughout their 



