INFLUENCE OF LIGHT ON PROTOPLASM 371 
the surface so as to receive ag little as possible. These 
two conditions are known as epistrophe and apostrophe 
respectively. When the conditions of the incidence of the 
light are altered, the chloroplasts change their positions 
accordingly. 
The Alga Mesocarpus exhibits the phenomenon in a very 
striking manner. It consists of somewhat oblong or slightly 
elongated cells arranged in a filament. Each cell contains 
a single band-like chloroplast which lies nearly parallel 
to the long axis of the cell. In ordinary daylight it places 
itself so that the surface of the band is exposed to the illu- 
minating rays, but if the light becomes intense, it revolves 
quickly upon its long axis, so that its edge is presented to 
them. 
A different effect of a strong light is manifested by many 
dorsiventral structures, of which the thallus of Marchantia 
affords a good example. Whichever side of the organ 
is brilliantly illuminated, the dorsal or upper surface shows 
accelerated growth, so that the thallus exhibits epinasty. 
Some of the radially symmetrical structures which have 
been mentioned as bilaterally organised (page 361) behave 
similarly. Such are runners of Polygonum aviculare, and 
other plants of similar habit. This phenomenon has been 
called photo-epinasty, as the increased growth of the dorsal 
side is due to the access of light. 
These facts may perhaps give us some idea of the influence 
of light upon protoplasm, and the condition of tone, one of 
whose chief features is the proper regulation of the per- 
meability of the protoplasm by water. In darkness meta- 
bolism and growth are greatly affected, the latter being 
unduly accelerated. In the presence of too strong a light, 
a deleterious influence is exerted. An intermediate con- 
dition exists in which the vital processes of growth and 
nutrition and the sensitiveness to external influences are 
seen at their best. This is the condition of tone or photo- 
tonus, and its maintenance may be spoken of as due to the 
tonic influence of light. It is frequently said that light 
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