INFLUENCE OF LIGHT ON PROTOPLASM 371 



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 ohlong 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 

 dor si ventral 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 



24* 



