582 BIOLOGICAL EFFECTS OF RADIATION 



COLONIAL FORMS 

 VOLVOX 



Volvox is a slightly elongated globular colonial organism somewhat 

 less than 1 mm. in diameter. It consists of numerous cells (zooids) 

 each of which contains two flagella and an eyespot. The zooids are so 

 arranged as to form a single layer at the surface of the colonies. The 

 eyespot in each zooid is directed toward the posterior end of the colony, 

 but those at the anterior end are much larger than those at the posterior 

 end. 



The colonies rotate on the longitudinal axis as they swim. This is 

 due to the diagonal stroke of the flagella. In a beam of light they usually 

 orient and go fairly directly either toward or from the light, i.e., they 

 may in a beam of light be either photopositive or photonegative or 

 neutral. 



If they are swimming toward the light and the intensity is rapidly 

 decreased without any change in the direction of the rays, rotation on 

 the axis stops and forward movement increases greatly, but this con- 

 tinues for only a few seconds. If the intensity is increased, forward 

 movement stops and rotation increases. If they are swimming from 

 the light the reverse occurs, i.e., forward movement decreases if the 

 intensity is increased and increases if it is decreased. If the colonies are 

 neutral, there are no such responses to changes of intensity. These 

 responses consist chiefly, if not entirely, of rapid changes in the direction 

 of the stroke of the flagella. In photopositive colonies rapid decrease in 

 illumination causes the stroke to change from diagonally backward to 

 straight backward and increase in illumination causes it to change from 

 diagonally backward to sidewise. In photonegative colonies precisely 

 the reverse obtains. If the luminous intensity is slowly changed these 

 responses do not occur. They are therefore dependent upon the rate of 

 change of intensity, i.e., they are shock-reactions which are somewhat 

 similar to those observed in Euglena. 



If Volvox is kept in low illumination or in darkness for several hours 

 it becomes inactive, and if the illumination is now increased it gradually 

 becomes active again. These responses consist chiefly, if not entirely, 

 in changes in the rate or the efficiency of the stroke, not in changes in the 

 direction of the stroke of the flagella. They are relatively slow responses 

 which occur even if the luminous intensity is gradually changed. They 

 are primarily dependent upon change in luminous intensity, not upon the 

 rate of change. There are consequently in Volvox two different types of 

 response, typical shock-reactions and responses which are often called 

 kinetic responses. 



If a colony of Volvox in a beam of light is laterally illuminated, it 

 turns gradually until it is oriented and then proceeds either toward or 



