CHAPTER VIII. 



ALGAE. 

 CHLAMYDOMONAS. 



375. The greenish, or sometimes reddish, colour of 

 standing rain-water (in tubs, gutters, ponds, puddles, and 

 ditches) is due to various minute organisms, among which 

 are species of Chlamydomonas and Haematococcus (Sphae- 

 rella). These two genera are readily distinguished, though 

 closely allied. 



(a) Mount a drop of water containing actively motile 

 Chlamydomonas. All that can be seen with the low power 

 is a number of minute green specks swimming rapidly 

 through the water in all directions. If possible, find one 

 which is stationary, and with the high power note that the 

 plant is unicellular, consisting of a pear-shaped cell, in 

 which may be distinguished (1) a thin but distinct cell- 

 wall ; (2) two fine threads (cilia) at the pointed end 

 these may be still seen waving from side to side ; (3) a bell- 

 shaped chloroplast, open towards the anterior pointed end 

 of the cell; (4) a small bright speck, the pyrenoid, lying 

 in the chloroplast at the broad end of the cell ; (5) the 

 nucleus, lying in the protoplasm within the chloroplast, at 

 the centre of the cell ; (6) two minute clear vesicles (con- 

 tractile vacuoles) at the pointed end of the cell ; (7) a 

 red dot of pigment (" eye-spot ") lying at one side just 

 behind the vacuoles. 



(b) Place a drop of chlor-zinc-iodine on a slide, add a 

 drop of water containing Chlamydomonas, cover, and with 

 the high power note (1) that the reagent has killed and 



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