INFLUENCE OF LIGHT ON PROTOPLASM 363 
surfaces to the light. In the first case the chloroplasts lie 
parallel to the surface of the leaf, and receive as much 
light as they can; in the last they lie at right angles to 
the surface so as to receive as 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. Hach 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 illuminating 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 
353) behave similarly. Such are runners of Polygonum 
aviculare, and other plants of similar habit. This pheno- 
menon 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 permeability of the protoplasm by water. In dark- 
ness metabolism 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 
condition exists in which the vital processes of growth and 
nutrition and the sensitiveness to external influences are 
