cells may be adjoined or merely inclosed by a colonial mucilaginous 

 envelope. In a few forms the cells are connected by strands formed 

 by the remains of old mother-cell walls. The chief characteristic 

 which all forms have in common is a negative one, namely, the 

 inability to multiply by cell division in the vegetative state. Auto- 

 spores are common in vegetative reproduction, as is also the habit 

 of forming daughter colonies within each cell of a mature colony. 

 Zoospores are used by some forms in asexual reproduction and 

 isogametes are the rule in the genera which have sexual reproduc- 

 tion. 



The cells vary widely in shape, being globose, ovate, acicular, 

 fusiform or polyhedral. Although most forms are uninucleate, a few 

 are coenocytic and hence are regarded by some phycologists as the 

 likely ancestors of the Siphonales. The form of the chloroplast varies 

 almost as much as the cell shape. There may be numerous ovate 

 discs, parietal plates, networks, or in some genera cup-shaped 

 chloroplasts. Pyrenoids may be lacking, but usually there are 1 to 

 several. One family, the Endosphaeraceae, is unique in being diploid 

 in the vegetative state. 



Key to the Families 



1. Unicellular, relatively large, and irregular in shape; wall very 

 vi'ide, lamellated, and not uniform in thickness; inhabiting 

 the tissues of higher plants, or free-living endosphaeraceae 



1. Unicellular or colonial; cells varied in shape but not irregular; wall 

 of uniform thickness and not definitely lamellated; free-living 



or attached, rarely subaerial 2 



2. Free-floating, or adherent on soil 3 



2. Attached and sessile, unicellular; cells fusiform characiaceae 



3. Cells cylindrical and forming a macroscopic network, or 

 triangular or polyhedral and united to form either a flat 



and circular, or globose coenobium (colony) hydrodictyaceae 



3. Cells not cyUndrical, and not forming colonies as above — 4 



4. Unicellular, solitary or sometimes gregarious, free-floating (usually 

 on moist soil if adherent), reproducing by zoospores (rarely aplan- 

 ospores) which do not adhere to one another but which are liber- 

 ated separately from the parent cell (See Chlorella 



however ) chlorococcaceae 



4. Colonial, or solitary not reproducing as above 5 



5. Thallus a hollow, globular coenobium of spherical or 

 pyramidal cells, adjoined to neighboring cells by 



processes, rarely by direct contact— coelastraceae 



5. Thallus not as above 6 



6. An irregular mass of mucilage, often darkly colored, 

 containing ovate or spherical cells at the periphery 



(compare with Dimorphococcus in the Oocystaceae) botryococcaceae 



6. Not as above 7 



[210] 



