410 F. W. GAMBLE AND FREDERICK KEEBLE. 



beach decreases^ the rate of flow of the drainage falls off. 

 A point is reached at which Convoluta is able to make 

 headway against the slower flow, or by cohering in masses to 

 divert it, and hence to form a new patch. Thus a seaward 

 bound will be set to the Convoluta zone, and animals carried 

 further seaward will be destroyed, or returned by the rush of 

 the incoming waters to their former place. That these essays 

 in distribution, now landward, now seaward, are continually 

 going on, may be readily appreciated. A heel-mark in the 

 neighbourhood of the patches forms a little lake into which 

 the drainage water flows, and which soon becomes filled with 

 the dark green scum of Convoluta. 



2. The Factors that regulate the Formation and 

 Position of the Colonies. 



The influence of light in determining the distribution of 

 Convoluta is indirect rather than direct. Previous writers, 

 Haberlandt (1891) in particular, have supposed that the 

 phototi'opism of Convoluta is of direct value in enabling 

 this animal to place itself in the area of brightest illumination. 



The manifestations of phototropism are so striking in the 

 laboratory that one is tempted to asci'ibe too high an im- 

 portance to it. In the laboratory, unilateral illumination of 

 a vessel containing Convoluta causes a rapid congregation 

 of the animals about the brighter side. Yet in the open a 

 patch of Convoluta may lie tranquilly in a shadow whilst, 

 round about, the ground is bi-ight with sunlight. The 

 explanation of this seems to us to be that the state of tone, 

 compounded of autonomous and light-induced tone, closely 

 determines pliotolro])ism. In the laboratory, the tone of the 

 animal is interfered with, and phototropic response, suffering 

 no inhibition, manifests itself with the " certainty of a reflex 

 action ; " but in the natural state the reflex action is not 

 certain, a stimulus has to lodge its appeal before the tribunal 

 of tone, and its petition may or may not be granted. Whether 

 a light-stimulus will produce a tropism or not will depend 

 not only on the nature of that stimulus, but also on the 



