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. Immediately inside the cell-wall 

 and closely applied to it is a layer of protoplasm, to which 

 the name primordial utricle is applied. In Spirogyra this 

 is so thin that it is difficult to make out under natural con- 

 ditions except with a very high power. It can however be 

 easily demonstrated by the use of what are termed plas- 

 molysing 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 Pro- 

 tococcus. 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 characteristic 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 



