DESMIDIE.E 447 



zygospore is formed in the conjugation-canal, and not 

 in the cavity of one of the conjugating cells ; (2) that 

 a part only of the protoplasm of the conjugating cells 

 is used up in forming the zygospore. 



DESMIDIE.E 



Members of the closely-allied group of the Desmidiece 

 should alse be examined ; e.g. species of Closterimn, 

 which is not uncommonly to be found in standing 

 pools of fresh 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 

 striaB, about the equatorial part of the cell. 



3. The colourless protoplasmic basis, which includes 



a. Two large chromatophores of equal size, 



disposed symmetrically on either side of a 

 central clear space. 



b. 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. 



