LECTURE V. 



CHLAMYDOMONAS is a very common object, but it is confused 

 usually with so many other organisms bearing but a superficial 

 resemblance to it that none of these objects has a special name in 

 our language. Stagnant water often shows a green coloration ; 

 one of the commonest causes of this greenness is the presence of 

 Chlamydomonas in the water, but it may also be due to many 

 other organisms. 



If one takes a drop of this green water, which may be found in 

 any stagnant pool, ditch or rain pipe, and examines it microscopi- 

 cally, the greenness will be seen to be due to a vast number of 

 tiny green specks, some of which are stationary, some are buzzing 

 about and some are almost at rest but jerk about in an uneasy 

 fashion. 



First it will be well to apply the Ghost-micrometer. It shows 

 that these little green specks are some four or five times as long 

 as a yeast cell. They have somewhat the same shape. They are 

 egg-shaped. Immediately we endeavour to make out details as 

 to their structure we become impressed with the rapidity of their 

 motion, which makes it very hard for us to observe the cell 

 closely. By means of the Ghost-micrometer and a stop-watch it 

 is easy to determine that the cell travels at the rate of about cri 

 mm. in 2 sec. or about i mm. in 20 sec., which is about the 

 same speed as the tip of the minute hand of a large watch. The 

 speed is by no means uniform, and much higher and lower speeds 

 may be registered. 



For observation of the structure we must select a cell at rest. 

 In a good culture a comparatively small number will be found com- 

 pletely motionless, for many of those which remain quiescent for a 

 time will occasionally betray signs of their restlessness by a spas- 

 modic oscillation from side to side. One of these occasionally 

 moving cells is the best for examination, inasmuch as its motions 

 usually rotate the cell, and turn different aspects to the observer. 



Like that of yeast, this cell is evidently bounded by a cell-wall, 

 and in this case its thickness may often be seen by careful focussing . 



35 3* 



