94 pringsheim's researches on chlorophyll. 



ration of the water in the drop, by the presence of many animal 

 and vegetable organisms alongside of the specimen examined 

 — Flagellata, Radiolaria, Sec, moving qnietly about, often in 

 the illuininated area, without suffering any harm — and, lastly, 

 by the exemption from the action of the cells adjacent to those 

 exposed. Moreover, in blue and green light the temperature 

 never can reach any high degree (see pp. 87 and 88) . 



Within the cells themselves, too, where action of light is 

 manifested, no purely heat effect, such as might be pro- 

 duced by invisible heat rays, occurs. In conformity with 

 our knowledge of heat dic^tribution in the spectrum, and of 

 diathermancy of elementary gases and liquids, the tempera- 

 ture determinations already given (pp. 87 and 88) show that 

 when the light passes through a solution of iodine in bisul- 

 phide of carbon (spectra y or S, fig. 28), much more heat 

 is developed than when under like conditions the light 

 passes through a solution of copper chloride and ammonia 

 copper sulphate (spectra ^, tj, B, i, k, fig. 28). In any red 

 light the temperature in the water drop is raised within five 

 minutes to over 45° C, and yet after fifteen to twenty 

 minutes of exposure there is neither decolorisation nor 

 death of the cell. On the other hand, in green and blue 

 light, cells are decolorised and killed in five minutes, al- 

 though after fifteen to twenty minutes uninterrupted exposure 

 the temperature scarcely reaches 35° — 36° C. — a degree 

 quite harmless to the cells. Now, although these tempera- 

 tures in the drop and the illuminated area are not an exact 

 measure of the temperature reached within the cells them- 

 selves, and it may therefore hardly be admitted that in the 

 case of the green and blue light the changes are evoked by 

 a temperature, at the highest, of 35°, yet the circustance that 

 the warm red rays have no effect, whilst the cold green and 

 blue ones are active, shows that in producing these changes in 

 the cell the intensity of light effect of the rays is not propor- 

 tional to their heating power. The destruction in the pro- 

 toplasm which leads to the death of the cell depends, there- 

 fore, not alone upon the thermal activity of the rays, but is 

 to be referred rather to their chemical activity, in which 

 colour is of great significance. How far the light affects the 

 chlorophyll colouring matter alone is subsequently discussed, 

 but in passing it may be noted that the inert red rays are 

 more strongly absorbed in the chlorophyll colouring matter 

 (compare spectra (3 with y and B) than the active green. 



The relation of the action of light to the chemical pro- 

 cesses in plant cells is made more clear when the plants 

 exposed to the intense light are placed in different gases or 



