PLANTS. THALLOPHYTA 935 



a. Algcc. 



Algae, or seaweeds, are thallophytes characterized by the pres- 

 ence of Chlorophyll, or leaf-green, though the color is by no means 

 always green. They are largely aquatic in habitat, most of the more 

 striking forms occurring in the sea. According to the prevailing 

 color, three divisions are made — the Green Algae, or Chlorophycccc; 

 the Brown Algae, or Phccophycecc; and the Red Algae, or Rhodo- 

 phycecc. The Cyanophycece, or blue-green algae, are also frequently 

 included under the algae. 



The Clilorophycecc include three forms in which the chlorophyll 

 is not accompanied by other coloring matter. With the typical 

 green alg^e (EucJtlorophycecc) are generally included the divisions 

 Conjugatacecc and Characecc, which have a separate phyletic stand- 

 ing. The common green sea-lettuce, Ulva, is a good example of an 

 expanded form, but in many of the green algae (especially the Con- 

 fervales), the thallus consists of filaments, branched or unbranched, 

 attached at one extremity and growing almost wholly at the free 

 end. Some forms (Halimeda, Acetabularia, etc.) are encrusted 

 with lime and are important on "coral" reefs. The Pond-scums, 

 or Conjugatcc (so named from their method of reproduction) in- 

 clude the Desmids, which have the power of independent move- 

 ment. 



Quite distinct from the others are the Characecc, the most highly 

 differentiated of the green algae. Of these the common stonewort, 

 Cliara, growing in fresh water lakes, is the typical form. This is 

 attached to the bottom of pools by rhizoids and grows upward by 

 means of an apical cell forming a pointed axis, which gives off 

 whorled appendages at regular intervals. Long branches occur in 

 each whorl, and these give off secondary whorls of jointed ap- 

 pendages. The distance between the nodes from which the ap- 

 penflages arise may be several centimeters. All are encrusted with 

 lime. The reproductive organs are also highly differentiated. 

 Antheridia and oogonia are formed at the nodes of the appendages. 

 The egg cells, or oogonia, when ripe are surrounded by five spirally 

 twisted cells, and crowned by a circle of smaller ones, which after- 

 ward separate to allow fertilization. The outer cells become very 

 hard and calcareous and are extensively preserved, in some cases 

 contributing to the formation of limestones. 



Over 1.600 species of true green algae are known. The Pond 

 Scums, or Conjugatae, add nearly 1,300 species more, while the 

 stoneworts, or Characeae, are represented by only about 180 species, 

 making a total of over 3,000 species. 



