446 PHYSIOLOGY 



cycads, exhibit ciliary locomotion. The cilia are so slender, and when 

 magnified sufficiently their movements are so rapid, that the details of 

 the strokes are difficult to follow. In the thicker cilia of infusoria the 

 forward stroke (fig. 679) consists of a progressive bending, which begins 

 below the free tip and advances to the base, where it is most powerful. 

 At the moment of greatest efficiency (fig. 679, 2), the curve bears 

 against the water like the blade of an exaggerated spoon oar (though, 

 of course, the cilium is not flattened). The return stroke (fig. 680) is 



slower and consists of a reverse and some- 

 what different curvature, advancing from 

 base to apex. 



Cause. — The cause of these repeated 

 lashings is completely hidden. They con- 

 tinue for a time and then cease. Though 

 they cannot be initiated, they can be 

 stopped or modified in rate by appropriate 

 680 stimuli, and their duration can be pro- 



FiGs. 679, 680. — Diagram- longed. Thus, if zoospores of algae be 

 matic representation of sucessive released in light, they may swim about 



positions (as numbered) of ciha "^ ^ 



of Vrostyia grandis; 679, in for- lor a few hours, then attach themselves 

 ward stroke; 680, in recovery, and germinate. But if they be kept in 

 — After Ver WORN. darkness, the swimming may continue for 



two or three days, until the zoospore seems entirely exhausted and 

 perishes without settling down. 



Taries. — The direction of swimming may also be controlled by ex- 

 ternal agents. The phenomena of directed locomotion are compre- 

 hensively called taxies, and with a prefix, designating the directive agent, 

 we have phototaxy, thermotaxy, chemotaxy, etc. These responses, 

 apparently simple, are really very difficult to interpret, and experiments, 

 seemingly quite conclusive, may lead to false inferences through the 

 operation of some overlooked factor. Thus, if a dish containing zoo- 

 spores of algae be placed on a window ledge so that one side is more 

 brightly illuminated than the other, the swarm spores will be seen to 

 accumulate on the side with brighter light, and this movement was 

 described at first as a positive response to light. Later it was found 

 that the droplets in an oil emulsion would behave in the same way 

 because of the previously unnoticed differences in temperature, making 

 convection currents in the dish. Two factors were therefore involved 

 and more rigid tests were needed to demonstrate photota^. 



