45 
PHYSIOLOGY 
difficult to determine whether this response is due to the direction of the 
light rays, or to the fact that one region is more brightly illuminated than 
another. Accumulation certainly occurs in regions of moderate light, 
with avoidance of the more shaded or the more brightly illuminated 
portions. The most exact of the recent studies of Volvox shows that 
its orientation is controlled by the relative intensity of the illumination 
on different sides of the colony, and as it swims with a definite pole 
forward, swimming after orientation causes it to move nearly parallel 
with the rays, some deflections from this course being due to certain 
minor disturbing factors. 
In phototaxy, as in chemotaxy, organisms respond both by orientation 
and by recoil, though, so far as known, the latter is much less common. 
The light waves vary in action according to their length, the reds and 
yellows, though the brightest, being quite unstimulating, whereas the 
blues are most effective. Yet this gives no clew to the real nature of the 
excitation or of the organs by which it is perceived. 
Geotaxy. Certain organisms have also been found to be geotactic. This prop- 
erty is quite distinct from others; for organisms that respond alike to other stimuli, 
such as light and oxygen, may react differently to gravity, the one being positively, 
the other negatively geotactic. Upon such irritability may depend the al ility of 
the creatures to rise or sink through the water on occasion. 
Motion of cell organs. Not unrelated to the movements of free- 
swimming organisms that have been described are the movements of 
organs of the cell which take place within the 
limits set by the wall. Such, particularly, are 
the movements of the chloroplasts and the nu- 
cleus. The former are known to be in part 
responses to light stimuli. Certain algae of 
the genus Mougeotia (Mesocarpus) have a 
single platelike chloroplast, which lies in the 
axis of the cell, facing the incident light, when 
this is of appropriate intensity. But if the 
light becomes more intense, the plate rotates 
until the edge is presented to the light. The 
numerous rounded chloroplasts of seed plants, 
FiGS.68i, 682. Two leaf 
cells of a moss (Atrichum mosses, etc., alter their distribution and their 
undutatum) seen from above: shape according to the illumination (figs. 681, 
the chloroplasts in 68 1 in ,-0 j T* _^ TTT / 
epistrophe; in 68 2) in apos- 682 > and m Part HI > fi g s ' 75, 759)- This 
trophe. After SCHIMPER. suggests a sort of escape from too bright light, 
