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, 

 mosses, etc., alter their distribution and their 

 shape according to the illumination (figs. 68 1, 

 * 682, and in Part III, figs. 758, 759). This 

 trophe. — After Schimper. suggests a sort of escape from too bright light, 



Figs. 681, 682. — Two leaf 

 cells of a moss (Atrichum 

 undulatum) seen from above : 

 the chloroplasts in 68 

 epistrophe; in 682, in apos 



