Practical Plant Biology. 



of the equatorial optical section. Chemical tests show that this 

 cell-wall is of cellulose. 



The space inside the wall is evidently filled with a granular sub- 

 stance which at first sight appears suffused with a brilliant green 

 colour. More precise observation shows that at one pole of the 

 cell there is a small beak-like prominence which is free from the 

 green colour. Here also it may be seen there is no cell-wall, and 

 the beak is plainly formed of the internal protoplasm of the cell 

 projecting through an orifice in the cell- 

 wall. By very careful manipulation of 

 the fine adjustment, condenser, dia- 

 phragm and mirror you will be able to 

 see one or two excessively fine and 

 quite transparent threads extending out 

 from the beak away from the cell. 

 These threads are called cilia. They 

 are fine extensions of the protoplasm 

 into the surrounding water, and are 

 about once and a half, or twice, as long 

 as the cell. In quiescent specimens 

 one cilium is usually comparatively easily 

 seen. It is used to anchor the cell and 

 is in contact with, and adherent to, the 

 surface of the cover-glass or slide. Be- 

 ing at rest it 'is merely a matter of 

 careful focussing and adjustment of the 



light to make it visible. The second 

 ciUum be fixfid Hke ;tg f u i 



. . J . . 



lts own adhesiveness to the glass. More 



chloroplast ; e, nucleus ; /, often, however, it retains its freedom 

 pyrenoid; g, central cyto- an( j lashes or waves to and fro. This 

 plasm; h, outer cytoplasm; mQtion makes [ts observation exceed- 

 ingly difficult and, while free, only oc- 

 casional glimpses of it can be caught, 

 when its motion changes or when it pauses for an instant. Evi- 

 dently these cilia are the organs which propel the cell through 

 the water in the way already observed. The protoplasm com- 

 posing the cilia seems quite homogeneous, it has a higher re- 

 fractive index than the surrounding water and hence shows that 

 pale, bluish coloration already observed in the protoplasm of yeast. 

 Otherwise the cilia are colourless. 



Turning our attention once more to the body of the cell we can 

 make out the outline of a more or less spherical shape lying in 



M-H-D 



FIQ. 7. Chlamydomonas, x 

 125 a, cilia; b, proto- 

 plasmic prominence ; c, 

 contractile vacuole ; d, 



cell-wall; 

 eye-spot. 



k, stigma or 



