Mast, Light Reactions in Lower Organisms. 123 



anterior and the posterior half of the body, and since this orienta- 

 tion to gravity is definite only in specimens containing daughter- 

 colonies it is evident that the daughter-colonies, located as they 

 are mostly in the posterior half of the body, render it heavier than 

 the anterior half. 



The specific gravity of living Volvox is also greater than one. If 

 the colonies become inactive they sink to the bottom, and it is 

 undoubtedly due to this that they are frequently found lying 

 quietly on the bottom of the aquarium after being in darkness all 

 night. The fact that the specific gravity of living colonies is 

 greater than one and that the posterior end of those wiiich contain 

 daughter-colonies or spores is heavier than the anterior end, has 

 an important bearing on orientation to light and the direction of 

 motion. 



It is owing to the difference in weight of the two ends, that the 

 anterior end turns up, if for any reason the forward motion of a 

 colony ceases. In this position the colonies are frequently found 

 in very dim light, apparently hanging in the water motionless. If 

 they become active while in this position, thev swim upward. 

 Such activity may be induced by light so dim that the organisms 

 do not orient. The degree of activity in light of low intensity, 

 without doubt, depends upon the physiological state of the organ- 

 ism, for it was frequently noticed that many colonies did not become 

 quiet in darkness, and several times after exposure to darkness 

 for as long as four or five hours, a large majority was found at the 

 surface of the water, apparently clinging to the surface film. 



If horizontal movement of Volvox colonies toward a given source 

 of light is observed from the side instead of from above, as was 

 customary m the experiments described in the preceding pages, 

 it can be clearly seen that the longitudinal axis of most of the speci- 

 mens forms a decided angle with the bottom of the aquarium, that 

 is, the posterior end is lower than the anterior. This angle varies 

 from zero to 90 degrees. Contrary to the observations of Klein 

 ('89, p. 169), it was found to be larger in organisms which contain 

 numerous daughter-colonies and spores than in those which do 

 not contain these structures. It is therefore in all probability 

 caused by the difference in weight of the two ends. 



The angle which the axis makes with the bottom of the aquarium 

 varies also with the light intensity. The more strongly positive 

 a given colony is, the smaller the angle; but the positiveness of 



