Smith—Monograph of Scenedesmus. 425 
tice has been followed in the treatment of the different 
species. The cultures also show that different forms may 
possess the same character (as spines at the four corners) 
but have dimensions which differ greatly within certain fixed 
limits. I have therefore considered as varieties strains show¬ 
ing the same external morphological characters but varying 
in the size of the component cells. An attempt had been 
made throughout the work to make the descriptions suit¬ 
able to the needs of the investigator who collects his mate¬ 
rial in the field, and only those characters have been in¬ 
cluded which are discernable through the microscope. 
The dimensions of the cells err on the side of exaggeration 
if at all. The work of Senn, Chodat, and myself has shown 
that external conditions influence the size of the cell to some 
extent, so control cultures have been carried out in sterile 
lake water to avoid the abnormalities produced by a too 
high strength of the nutrient solution. Unless otherwise 
noted the cultures have been made in 0.1% Knop’s solution 
since I find that I cannot distinguish between the cultures 
grown in this medium and in natural waters. 
Key to the Species qf Scenedesmus. 
I. Colony enclosed in a gelatinous sheath. 
1. S. oahensis (Lemm.) nov. comb. 
II. Colony not enclosed in a gelatinous sheath. 
1. Cell membrane smooth, without terminal spines, 
teeth, granulations, or lateral ridges. 
A. Cells acicular to spindle shaped. 
a. Cells without polar bulbs or thickenings. 
X. Cells in a flat plane. 
*. All cells in the coenobe erect. 
2. S. obliquus (Turp.) Ktz. 
**. Median cells erect, outer cells curved. 
3. S. dimorphus (Turp.) Ktz. 
XX. Cells not in one plane. 
*. Cells alternately arranged, terminal cells 
usually not in same plane as others in 
coenobe. 
4. S. Bernardii nov. spec. 
**. Cells forming a curved surface. 
5. S. acuminatus (Lag.) Chod. 
