6 9 



CHAPTER V. 



UNICELLULAR PLANTS PROTOPHYTA. 

 PROTOCOCCUS. 



As an example of the simplest form of plant known, we may 

 select Protococcus pluvialiS) one of the organisms which gives 

 the green colour to damp wood and stones, and which is 

 found also in large quantities wherever rain-water collects. 



The plant is of extremely minute size, varying from 

 ToiioT^h to 47j th of an inch in diameter. In shape it is 

 ovoid or elliptical, and presents, when examined under a 

 high magnifying power, a differentiation into protoplasm 

 and cell- wall. The protoplasm has diffused through its mass 

 or aggregated in the form of rounded masses the green 

 colouring matter chlorophyll. In addition there may be 

 seen a nucleus and granules. Some forms have, in addition 

 to the chlorophyll, red colouring matter present, usually in 

 the form of a spot at one end. The cell-wall is of an appre- 

 ciable thickness, is colourless, and composed of cellulose. 



Occasionally Protococcus becomes motile, and migrates 

 from place to place in the medium (water) by means of the 

 vibration of two flagella or cilia which are extruded from 

 one end of the cell. Careful examination under a high 

 magnifying power shows that the protoplasm has contracted 

 generally from the cell-wall, but is continuous with the cilia. 

 Not infrequently the cell-wall becomes entirely lost, and 

 Protococcus moves about as a naked mass of protoplasm 

 provided with the two cilia already mentioned. The nucleus 

 and red spot are distinctly visible at this stage, usually close 



