364 VEGETABLE PHYSIOLOGY 



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. 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 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 

 this 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 

 354) 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 



