PROTOPLASM 11 



a few important facts regarding them at this point. Plants are 

 probably the most abundant of all living organisms. They range 

 in size from microscopic, unicellular species to the giant Red- 

 woods of California. Four major divisions, or phyla, of the Plant 

 Kingdom are recognized, namely : (a) the Thallophyta, which 

 include (i) the Algae (e.g., Pleurococcus and Spirogyra, noted 

 below) and (ii) the so-called Colorless Plants, or Fungi (e.g., 

 Racteria, Yeast, and Molds, noted below), which are unique in 

 that they are the only group of plants which lack chlorophyll ; 

 (b) the Rryophyta, or Mosses ; (c) the Pteridophyta, or Ferns ; 

 and (d) the Spermatophyta, or Flowering Plants, which include 

 most of our common species. Three common forms of green 

 plants may now be indicated. 



Pleurococcus. This is a very common, unicellular Alga which 

 forms a greenish covering on the bark of trees, on flower pots, and 

 on many other surfaces where a suitable environment is provided. 

 The greenish covering, when examined microscopically, is found to 

 consist of great numbers of tiny cells, each of which, since it con- 

 tains the basic food-forming substance, chlorophyll, is able to 

 manufacture complex foodstuffs photosynthetically from the simple 

 inorganic materials present in the immediate environment, and thus 

 to carry on an independent existence. Here in this minute cell, 

 then, although it lacks all the specialized structures of the higher 

 plants, are concentrated all the characteristic and essential life 

 processes of green plants, including photosynthesis. (W. pp. 30- 

 34.) 



Spirogyra. This is a simple type of multicellular Alga which 

 is frequently found floating on the surface of stagnant fresh-water 

 ponds, and is often referred to as the Pond Scum. Spirogyra con- 

 sists of a variable number of long, tubular cells attached end to 

 end to form a fine thread-like filament, which constitutes the 

 entire plant body. The cells, although attached, are independent 

 functional units, each one performing all the essential life proc- 

 esses just as is the case in Pleurococcus. The cells of Spirogyra 

 are characterized structurally by a very large and beautiful, spiral 

 chloroplastid which lies just under the cell wall embedded in a 

 layer of cytoplasm, like a piece of green ribbon. There may be 

 one or more of these chloroplastids in each cell, depending upon the 

 particular species examined. Each chloroplastid contains not 

 only the chlorophyll for photosynthetic food manufacture, but 



