388 VEGETABLE PHYSIOLOGY 
not only a slowing of the responsive movement of the stem, 
but in some cases an actual reversal of it. Both movements 
seem to have for their object the placing of the shoot in a posi- 
tion most favourable to the plant under the then present con- 
ditions of illumination, for too brilliant a light is deleterious. 
This reversal of the normal curvature in the presence of an 
excessive stimulation can be seen with great clearness in the 
case of certain sporangiophores of Phycomyces, as was first 
shown by Oltmanns. 
A somewhat similar response to the influence of a lateral 
light is exhibited by many unicellular organisms. When 
these are exposed to oblique illumination they take up a 
definite position with regard to the incident rays, placing 
their long axis parallel to them if the light is weak, and at 
right angles to them if it is intense. This behaviour is 
known as phototazis ; it is exhibited by the zoospores of 
any of the Algew and by certain Desmids. 
Before leaving the subject of the effect of a lateral light 
in inducing these movements, we may point out that the 
phenomena of heliotropism and apheliotropism must be 
distinguished from those of photo-epinasty and _ photo- 
hyponasty, which were alluded to in the last chapter (p. 871). 
The difference is easily seen, for in the latter cases the 
result of the access of the light is the same, whatever be 
the portion of the organ stimulated. The thallus of Mar- 
chantia becomes convex on the dorsal and concave on the 
ventral side, whether the light impinges on the one or the 
other. In the case of a heliotropic curvature the side 
which is stimulated always becomes concave; in that of 
an apheliotropic one the stimulated side becomes convex. 
Gravitation.—The force of gravitation exerts an influence 
upon plants which in some respects resembles that of lateral 
illumination. Most stems grow vertically upwards into the 
air; primary roots grow vertically downwards into the 
soil. A few organs, among which may be mentioned certain 
rhizomes and the runners of many plants, grow at right 
angles to the direction of gravity. When one of these is 
