DEVELOPMENT OF PLANTS 



173 



Some of these forms are free, some attached and others aggre- 

 gated in a mucilaginous mass. 



There are a great many of the stationary genera that show 

 the same habit of uniting into colonies that we noticed among 

 the motile genera. These colonies may be very simple and com- 

 posed of a few loosely aggregated plants forming flat discs or 

 star-shaped plates (Fig. 105, A). In other cases, we find spher- 

 ical and net-like colonies. The water net, Hydrodictyon, is a 

 common illustration of this latter arrangement (Fig. 105, D, E). 



Fig, 105. Colonial forms of unicellular green algae: A, Pediastrum, the 

 plants of the colony being arranged in a flat plate. B, a view of the outer 

 cells of the colony showing the formation of a new colony. C, one of these 

 new colonies. D, a plant of the water net containing a young colony. E, 

 enlarged view of one of the meshes of a net showing the geometrical arrange- 

 ment of the plants. 



These plants occur in ponds and sluggish streams, forming sac- 

 like nets three to four inches long, that are constructed in a 

 very regular manner. The reproduction is quite characteristic 

 of many of the Volvocales. Zoospores are formed in large num- 

 bers in any of the plants and after a period of motility, they 

 come to rest and arrange themselves into a new net inside of the 



