FLOWERLESS PLANTS 



177 



Pleurococcus. — The simplest of all green plants are the one-celled algae. 

 One of the commonest forms is the plant called Pleurococcus, which is 

 found in vast numbers on the bark of trees, on moist fence posts, on the 

 shaded parts of buildings and other moist localities.* 



If we gently scrape some of the green material from a bit of bark con- 

 taining these plants into a shallow dish containing water, and after a few 

 minutes transfer some of the material to a glass slide and examine it under 

 the high power of the microscope, something of the structure of the tiny- 

 plants can be made out. 



Plant Body. — The plant body of the Pleurococcus is found to crnsist 

 of a single very small green cell. This is surrounded by a rather prfjminent 

 wall of cellulose or woody material. Notice the protoplasmic contents of 

 the cell. Is the chlorophyll arranged in a definite manner as in the cell of 

 Spirogyra ? Notice that many of the cells are 

 joined together to form colonies. How many 

 cells do you find in such a colony? Do the 

 number of cells thus grouped together differ ? 

 Draw several colonies in different stages. In 

 such a simple plant as the Pleurococcus we 

 find no other method of reproduction plainly 

 seen except that by simple division. This _,, 

 is an asexual method known as vegetative ^^^^^fo'n'y'T^ou^r'clSrmei bl; 



reproduction. division of the original cell, A. 



Formation OF Zoospores. — Under some circumstances, however, certain 

 species of Pleurococcus are known to give rise to a cell which is capable of 

 movement in water by means of two whiplash threads of protoplasm which 

 protrude from one end of the cell. This motile cell is called a zoospore 

 (animal spore). It does resemble a very simple animal at this stage 

 of its existence. The spore thus formed is an asexual spore which may 

 swim away from the parent plant and found colonies in a new locality. 



Diatoms. — These plants are found in vast numbers living on the mud 

 or stones at the bottom of small streams. The plant body is inclosed in 

 a cell wall composed largely of silica 

 or glass. Many of the diatoms are 

 free-swimming. They compose a 

 large percentage of the living organ- 

 isms found near the ocean's surface. 

 The plant body is inclosed by cell 

 walls composed of two valves which 

 fit into each other like a box and 

 its cover. The cell color is brown. 



Diatoms are found as fossils, and 

 make up a large proportion of many rocks. The silicious skeletons in such 

 rocks are made of commercial importance, the rock forming a basis for 

 polishing powders. 



1 See Hunter and Valentine, Manwd, page 77. 

 hunter's BIOL. — 12 



Diatoms. 



