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.y 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 utride\=> 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 

 I^atiyj^_thjn_pariejtal. 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 cjiromatophores. 

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

 spoken of as a single chromatophore.J /Each cell may 

 therefore contain one chromatophore., or several* according 

 to the species, j 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 

 imbedded in the primordial utricle throughout their 



