SINGLE-CELLED PLANTS AND COLONIES. II 



lashes, called cilia, may be single at the ends of the cell (C, 

 fig. 9), or many at ends or sides {_A, fig. 9), or the whole cell 

 may be covered with them like hairs (^B, fig. 9). They may 

 be withdrawn or drop off when the plant comes to rest, as 

 when they form the scums previously mentioned. 



These plants are most interesting on account of their rela- 

 tion to health and disease, decay, fermentation, etc., which 

 cannot be discussed here.* 



17. Yellow-green algae. — Among the single-celled green 

 plants, one of the most common groups is that represented 

 by fig. 10, which shows one of a large series, in which the 

 body consists of a single cell with its wall, protoplasm, 

 nucleus, and a few relatively large chloroplasts. In this 

 greater specialization of the protoplasm, these plants show 

 the only advance upon the blue-green algse. The wall in 

 such as this Pleurococcus is almost uniform and quite thin. 

 The cells of some kinds are frequently associated in colonies, 

 embedded in jelly or not. 



EXERCISE III. 



Pleurococcus. — i. Examine with a lens pieces of bark bearing Pleuro- 

 coccus and similar algse. Note the irregular distribution of the green 

 granular heaps of plants. Is there any similarity to the distribution of 

 higher plants over uncultivated areas ? 



2. After soaking a. piece of bark for a few minutes, scrape off with 

 the nail or a dull knife blade some of the green material, spread it as 

 well as possible in a drop of water on a slip of glass, cover it with a 

 piece of thin glass, avoiding air-bubbles, and examine with a lens. 

 Observe the minuteness of some of the specks, which are mostly single 

 plants The larger ones are clusters of plants. 



3. Demonstration. Show a slide under microscope and have pupils 



* For further information on these plants, see Frankland: Our Secret 

 Friends and Foes ; Prudden : Story of the Bacteria, Dust and its Dan- 

 gers, Drinking-water and Ice Supplies ; Russell: Dairy Bacteriology; 

 Frankel {ir. by Linsley): Bacteriology (medical). 



