92 PRACTICAL BOTANY. 



which is not uncommonly to be found in stagnant 

 water. Mount specimens in fresh water, and examine 

 under a high power : note 



1. The more or less semilunar curved form of this 

 unicellular Alga. 



2. The smooth cell-wall, often marked by transverse 

 striae, which are indications of former cell-divisions. 



3. The colourless protoplasmic basis, which includes 



a. Two large chromatophores of equal size, 

 disposed symmetrically on either side of a 

 central clear space. 



I. A single nucleus, which occupies the central 

 space, but is sometimes difficult to observe 

 owing to the fact that the chromatophores 

 often extend across the central space, and 

 may even be connected. 



Observe further the peculiar plaited form of the 

 chromatophores, which also include several disk-shaped, 

 highly refractive bodies (pyrenoids). Stain with an 

 iodine solution : this will make the nucleus more ap- 

 parent, and the pyrenoids will stain a dusky purple. 



The plant multiplies by cell-division : try to observe 

 cells in which the process is going on. It also 

 conjugates, though this will not be readily observed. 



Other Desmids may also be compared, e.g. Micr aster ias, 

 which is to be found in peaty pools : it is of flattened 

 form with a deep median constriction, and stellate 

 outline : the pyrenoids are very well seen in this 

 species. For description of other forms of this family 

 reference must be made to books dealing specially with 

 the subject. 



