THE FOOD OF PLANTS. 51 



the light, they become green, their growth begins anew, and at 

 the end of the time starch is to be detected in the cells of the 

 leaves by macrochemical or microchemical tests. 



If mature leaves of vigorous pot plants of Tropaeolum or 

 Phaseolus, grown in the light, are examined maorochemically or 

 microchemically, starch will readily be detected in their cells. 

 If the plants are now left in the dark, at a high summer tempera- 

 ture for a short time (say forty-eight hours), at a lower tem- 

 perature for a longer time, the starch will disappear from the 

 mesophyll of their leaves. If the plants are again exposed to the 

 light for a few days, we can easily ascertain that their leaves are 

 once more rich in starch. It is a good plan in comparative in- 

 vestigations concerning the influence of illumination on the origi- 

 nation and disappearance of starch in the cells of the leaf tissue, 

 not to cut off entire leaves at the end of the separate periods, -but 

 pieces only. We are thus enabled to experiment with one and 

 the same leaf throughout the entire period of investigation. 



If we examine bud leaves of vigorous plants of Elodea cana- 

 densis, growing under normal conditions, we shall find starch 

 present in the cells in large quantities. If the plants are placed 

 in darkness, the starch completely disappears (in my experiments, 

 at a high summer temperature, often within twenty-four hours, 

 but certainly much more slowly at a lower temperature). Re- 

 newed illumination quickly causes reaccumulation of large 

 quantities of starch in the leaf tissue. I have found it advisable 

 not to treat the Elodea leaves with chloral hydrate and iodised 

 Potassium iodide solution till they have been extracted with boil- 

 ing water and hot alcohol for removal of their chlorophyll. 



At a high summer temperature filaments of Spirogyra are free 

 from starch after being kept in the dark for one to three days, as 

 is shown by testing with chloral hydrate and iodised Potassium 

 iodide solution. If we expose such starch-free filaments of Spiro- 

 gyra to direct sunlight, they very soon, e.g., even after half an 

 hour, contain large quantities of starch, in consequence of the 

 high temperature and the intense light. In diffuse light the 

 formation of starch proceeds far more slowly. 



To investigate the influence of light rays differing in refrangi- 

 bility on the formation of starch in chlorophyll, it is convenient to 

 employ as research material filaments of Spirogyra or Elodea 

 plants which have been deprived of starch by being kept for a few 

 days in darkness. The starch-free plants are placed under double- 



