44 6 
PHYSIOLOGY 
679 
680 
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 
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 thcy may swim about 
positions (as numbered) of cilia ' 
of Vrostyla grandis; 679, in for- 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. 
Taxies. 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 phototaxy. 
