366 BOTANICAL GAZETTE [MAY 
view with the pyrenoid most prominent. From this method of development 
it would appear that Platydorina is not a more specialized Gonium, but 
derived from ancestors of the Eudorina type. Platydorina is positively 
phototactic, exhibiting this habit to a conspicuous degree, but avoiding 
bright sunlight. The movement is always forward unless obstructions are 
met, and the rotation as a rule is from right over to left. This is the direction 
naturally favored by the resistance that the water offers to a moving body 
twisted in the manner of this coenobium. However, as the rotation in Pleo- 
dorina and Eudorina is predominately from right over to left, it is possible 
that the form of the colony in Platydorina is the result and not the cause of 
this rotation, Dr. Kofoid discusses the use of the term colony, as unfor- 
tunately inappropriate to the higher members of the Volvocacee, where 
there is a certain degree of cell differentiation, especially in Pleodorina and 
Carter’s form of Eudorina, with the probability of a physiological coopera- 
tion between the cells of Volvox. Which facts make this group of plants a 
most interesting one from a speculative and evolutionary standpoint, and 
perhaps we may pardon their failure to accommodate themselves to a system 
that makes possible the use of exact terms.—B. M. DAVIS. 
