416 A TEXTBOOK OF BOTANY [Ch. X 



mately become differentiated into egg cells and sperm 

 cells, which are formed in structures called respectively 

 oogonia and antheridia. On fertilization, oospores, which 

 are usually resting spores, are formed. These Algae spread 

 partly by their own locomotive powers, and partly by 

 diverse incidents of water movement, without special adap- 

 tations. They comprise five orders, — 



Order 1. Protococcales: the Unicellular Green Algm. 



Order 2. Confervales: the Confervoid Algm. 



OrderS. Conjugales: the Conjugate Algm. 



Order 4> Siphon ales: the Tubular Algm. 



Order 5. Charales : the Stoneworts. 



Order 1. Protococcales: the Unicellular Green 

 Algm. These are mostly microscopic forms dwelling in 

 fresh water or on land. Some are free-swimming by cilia or 

 flagella in the same animal-like manner as the Flagellates, 

 while others drift passively with the currents or remain fixed 

 by their gelatinous walls to rocks, not only in water, but in 

 damp shaded places on the land. 



A typical motile form is Chlamydomonas, common in fresh 

 water (Fig. 284). The single cell contains a large cup-shaped 

 chloroplast, in which is a protein body, the pyrenoid, com- 

 monly surrounded by starch grains, with the formation of 

 which it has something to do. A nucleus, red pigment spot, 

 and contractile vacuoles are also present, while locomotion 

 is effected by large terminal cilia, — the whole arrangement 

 recalling the Flagellates. It reproduces by longitudinal 

 fission and by formation of zoospores, which are miniatures 

 of the parent and have only to enlarge. This resemblance 

 between the zoospores of Algae and the adult Flagellates 

 holds true throughout the group. Other cells produce 

 gametes which conjugate in pairs to form resting zygospores, 

 whence later grow new plants. In certain species a bright 

 red pigment, of unknown function, produces a striking 

 appearance in mass ; and plants of this group (Chlamydomonas 

 nivalis), readily carried by the wind, produce the "red snow" 



