setae; sides concave, or in some slightly convex; chloroplast a 

 parietal plate covering most of the cell wall, or more massive in 

 old cells, with 1 pyrenoid. 



Polyedriopsis spinulosa Schmidle 1900a, p. 17 

 PI. 62, Figs. 2, 3 

 Characteristics as described for the genus; cells 12-25/x, in diameter; 

 setae 40/a long. 



Rare; in plankton. Wis. 



FAMILY SCENEDESMACEAE 



In this family the cells are adjoined to form definite patterns 

 and colonial aggregates of regular shape. This arrangement is 

 determined by the autospores which, when cut out of the parent 

 protoplast, become definitely related and oriented to one another 

 to form autocolonies in which there are always multiples of 2. These 

 colonies upon being liberated increase in size to form the mature 

 plant. 



There is a great variety of cell shapes (oblong, fusiform, spherical, 

 triangular, trapezoidal) and arrangement. The colony may be a 

 linear series, a flat plate, a trapezoidal aggregate, or a cluster of 

 radiating fusiform individuals. 



This family differs from the preceding one (Oocystaceae) in 

 that colonies are formed by the definite adjoining of cells in a 

 regular pattern. 



Key to the Genera 

 1. Cells ovoid, fusiform, crescent-shaped, or oblong, arranged with 



their long axes parallel to form a single 



or double series in 1 plane Scenedesmus 



( See S. acuminatus, with cells in a curved series. ) 



1. Cells of diflFerent shape, or otherwise arranged 2 



2. Cells fusiform or cylindrical, with long axes 



parallel; quadrately arranged in 2 tiers Tetradesmus 



2. Cells arranged otherwise 3 



3. Cells spherical, forming pyramidal and multiple 



colonies of 4; cells bearing long spines.— - Micractinium 



3. Cells not spherical, without spines or with but very short ones 4 



4. Cells with only their poles adjoined, extending in several planes 6 



( See Scenedesmus Bernardii, however. ) 



4. Cells, often in 4's, forming a flat coenobium with cells adjoined 



or not adjoined along their walls -- ^ 



5. Cells bearing spines.- - — - Tetrastrum 



5. Cells without spines— Crucigenia 



6. Cells cylindrical or fusiform -- Actinastrum 



6. Cells sausage-shaped, or crescent-shaped Tetrallantos 



[272] 



