CHAPTER VII 



THE FACTORS INVOLVED IN THE PROCESS OF ORIEN- 

 TATION IN COLONIAL FORMS 



I. Volvox glohator and minor 



Many interesting observations have been made on the 

 light reactions of Volvox since Leeuwenhoek discovered 

 this organism over two hundred years ago. The details in 

 the reactions have however only recently been worked 

 out. In 1907 I published an extensive paper on this sub- 

 ject, and the following account is based largely on this 

 paper. 



Volvox is an organism somewhat like a hollow sphere 

 slightly elongated. The largest colonies are nearly i mm. 

 in diameter and the smallest can readily be seen with the 

 naked eye. Each colony is composed of from 200 to 22,000 

 individuals and each individual consists of a single cell 

 known as a zooid. The zooids, interconnected with proto- 

 plasmic strands, are arranged side by side so as to form a 

 wall inclosing a cavity. They are very much like Chlamy- 

 domonas in structure and color. Each one contains tw^o 

 fiagella and an eye-spot which is situated on the outer pos- 

 terior surface. The eye-spots at the anterior end of the 

 colonies are from eight to ten times as large as those at 

 the posterior end much as in Pandorina represented in 

 Fig. 21. 



The colonies usually rotate counter-clockwise on the long 

 axis, like Euglena, but they seldom swim on a spiral course. 

 They orient and swim toward a source of light or away 

 from it in a general way; they do not however orient very 

 accurately. Colonies swimming horizontally toward a com- 

 pact source of light usually deflect either to the right or to 

 the left or up or down. The more strongly positive the 



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