ALG^ 



363 



But plants surrounded by water do not need such protec- 

 tion ; neither, since water is all about them, do they need 

 special structures for conveying- water from one end of the 

 plant to the other, as is the case with most land plants. 

 So it does not surprise us to find that the structure of 

 algae is very simple. 



A. Unicellular Forms. — The simplest algae are those 

 in which the individual plant is just one cell. Pleurococcus 

 is such a plant. Often you have seen it growing on the north 

 side of trees. The north side is more favorable than the 

 other sides because the sun 

 does not strike it, and hence 

 the bark is more likely to re- 

 main moist. You have seen 

 it also on old damp boards and 

 on wet rocks. It looks Hke 

 green paint. A Httle of it as 

 seen under the microscope is 

 represented in Figure 160. 

 Each cell is a separate plant. 

 Lacking the pressure of sur- 

 rounding cells, such cells tend 

 to assume the form of a per- 

 fect sphere. The pressure of 

 the water upon them is practically equal on all sides. They 

 illustrate perfectly the effects of osmosis upon an elastic 

 cell wall. (See page 102.) Reproduction is accomplished, 

 as the picture indicates, by the division of the whole body 

 of the parent. There is no such thing as the differentia- 

 tion of a part of the body for reproduction, such as we have 

 seen in the seed plants. 



Fig. 160. — Pleurococcus. Clusters 

 of the cells showing the manner of 

 reproduction by simple division 

 {fission). Highly magnified. 



